Legacy canvas should be disabled in GTK3 based builds (lp:#1803156)

[Original report](https://bugs.launchpad.net/bugs/1803156) created by **John Beard (john-j-beard)**
```
Whenever I start gerbview, it asserts endlessly (or at least very many
times!):

./include/wx/math.h(151): assert "x > INT_MIN - 0.5 && x < INT_MAX + 0.5"
failed in wxRound(): argument out of supported range

This is with wx-gtk3.

Stack (I can't see exactly what is going on as I don't have the debug
symbols to hand):

#0  0x00007f1716b5a25f in raise () at /usr/lib/libpthread.so.0
#1  0x00007f171841acfe in wxGUIAppTraits::ShowAssertDialog(wxString const&)
() at /usr/lib/libwx_gtk3u_core-3.0.so.0
#2  0x00007f1717f15282 in  () at /usr/lib/libwx_baseu-3.0.so.0
#3  0x00007f1717f154a9 in wxAppConsoleBase::OnAssertFailure(wchar_t const*,
int, wchar_t const*, wchar_t const*, wchar_t const*) () at
/usr/lib/libwx_baseu-3.0.so.0
#4  0x00007f17183e9021 in wxApp::OnAssertFailure(wchar_t const*, int,
wchar_t const*, wchar_t const*, wchar_t const*) () at
/usr/lib/libwx_gtk3u_core-3.0.so.0
#5  0x00007f1717f1577b in  () at /usr/lib/libwx_baseu-3.0.so.0
#6  0x00007f1717f1216b in wxOnAssert(char const*, int, char const*, char
const*, wchar_t const*) ()
    at /usr/lib/libwx_baseu-3.0.so.0
#7  0x00007f17184d1e33 in wxDCImpl::DeviceToLogicalYRel(int) const () at
/usr/lib/libwx_gtk3u_core-3.0.so.0
#8  0x00007f17184d6fa9 in wxGCDCImpl::Clear() () at
/usr/lib/libwx_gtk3u_core-3.0.so.0
#9  0x00007f170dd55db7 in wxDC::Clear() (this=0x7ffd519d1db0) at
/usr/include/wx-3.0/wx/dc.h:973
#10 0x00007f170dd54781 in GBR_LAYOUT::Draw(EDA_DRAW_PANEL*, wxDC*,
GR_DRAWMODE, wxPoint const&, GBR_DISPLAY_OPTIONS*) (this=
    0x564a870112e0, aPanel=0x564a85e90bd0, aDC=0x7ffd519d2220,
aDrawMode=GR_OR, aOffset=..., aDisplayOptions=0x564a86f36330) at
/home/john/src/kicad/gerbview/gbr_layout.cpp:161
#11 0x00007f170dd708d9 in GERBVIEW_FRAME::RedrawActiveWindow(wxDC*, bool)
(this=
    0x564a86f35aa0, DC=0x7ffd519d2220, EraseBg=true) at
/home/john/src/kicad/gerbview/draw_gerber_screen.cpp:74
#12 0x00007f170dfaea3a in EDA_DRAW_PANEL::ReDraw(wxDC*, bool)
(this=0x564a85e90bd0, DC=0x7ffd519d2220, erasebg=true)
    at /home/john/src/kicad/common/legacy_wx/eda_draw_panel.cpp:640
#13 0x00007f170dfae7c4 in EDA_DRAW_PANEL::OnPaint(wxPaintEvent&)
(this=0x564a85e90bd0, event=...)
    at /home/john/src/kicad/common/legacy_wx/eda_draw_panel.cpp:607
#14 0x00007f171808089e in
wxEvtHandler::ProcessEventIfMatchesId(wxEventTableEntryBase const&,
wxEvtHandler*, wxEvent&) () at /usr/lib/libwx_baseu-3.0.so.0
#15 0x00007f17180809a4 in wxEventHashTable::HandleEvent(wxEvent&,
wxEvtHandler*) () at /usr/lib/libwx_baseu-3.0.so.0
#16 0x00007f1718080cde in wxEvtHandler::TryHereOnly(wxEvent&) () at
/usr/lib/libwx_baseu-3.0.so.0
#17 0x00007f1718080d64 in wxEvtHandler::ProcessEventLocally(wxEvent&) () at
/usr/lib/libwx_baseu-3.0.so.0
#18 0x00007f1718080e02 in wxEvtHandler::ProcessEvent(wxEvent&) () at
/usr/lib/libwx_baseu-3.0.so.0
#19 0x00007f171862467d in wxScrollHelperEvtHandler::ProcessEvent(wxEvent&)
() at /usr/lib/libwx_gtk3u_core-3.0.so.0
#20 0x00007f1718080ba7 in wxEvtHandler::SafelyProcessEvent(wxEvent&) () at
/usr/lib/libwx_baseu-3.0.so.0
#21 0x00007f171842aa6b in wxWindow::GTKSendPaintEvents(_cairo*) () at
/usr/lib/libwx_gtk3u_core-3.0.so.0
#22 0x00007f171842af6c in  () at /usr/lib/libwx_gtk3u_core-3.0.so.0
#23 0x00007f171660973c in  () at /usr/lib/libgtk-3.so.0
#24 0x00007f17165b2c22 in  () at /usr/lib/libgtk-3.so.0
#25 0x00007f17161083d5 in g_closure_invoke () at
/usr/lib/libgobject-2.0.so.0
#26 0x00007f17160f5195 in  () at /usr/lib/libgobject-2.0.so.0
#27 0x00007f17160f85ed in g_signal_emit_valist () at
/usr/lib/libgobject-2.0.so.0
#28 0x00007f17160f9a80 in g_signal_emit () at /usr/lib/libgobject-2.0.so.0
#29 0x00007f17165bfca3 in  () at /usr/lib/libgtk-3.so.0
#30 0x00007f171639c55f in gtk_container_propagate_draw () at
/usr/lib/libgtk-3.so.0
#31 0x00007f171639c62e in  () at /usr/lib/libgtk-3.so.0
#32 0x00007f17164f200e in  () at /usr/lib/libgtk-3.so.0
#33 0x00007f17163a18dd in  () at /usr/lib/libgtk-3.so.0
#34 0x00007f17163a6717 in  () at /usr/lib/libgtk-3.so.0
#35 0x00007f17164f02d2 in  () at /usr/lib/libgtk-3.so.0
#36 0x00007f17165bfb3a in  () at /usr/lib/libgtk-3.so.0
#37 0x00007f171639c55f in gtk_container_propagate_draw () at
/usr/lib/libgtk-3.so.0
#38 0x00007f171640f6b2 in  () at /usr/lib/libgtk-3.so.0
#39 0x00007f171660973c in  () at /usr/lib/libgtk-3.so.0
#40 0x00007f17165b2c22 in  () at /usr/lib/libgtk-3.so.0
#41 0x00007f17161083d5 in g_closure_invoke () at
/usr/lib/libgobject-2.0.so.0
#42 0x00007f17160f499f in  () at /usr/lib/libgobject-2.0.so.0
#43 0x00007f17160f85ed in g_signal_emit_valist () at
/usr/lib/libgobject-2.0.so.0
#44 0x00007f17160f9a80 in g_signal_emit () at /usr/lib/libgobject-2.0.so.0
#45 0x00007f17165bfca3 in  () at /usr/lib/libgtk-3.so.0
#46 0x00007f171639c55f in gtk_container_propagate_draw () at
/usr/lib/libgtk-3.so.0
#47 0x00007f171639c62e in  () at /usr/lib/libgtk-3.so.0
#48 0x00007f171634c5e5 in  () at /usr/lib/libgtk-3.so.0
#49 0x00007f17163a18dd in  () at /usr/lib/libgtk-3.so.0
#50 0x00007f17163a6717 in  () at /usr/lib/libgtk-3.so.0
#51 0x00007f171634eed2 in  () at /usr/lib/libgtk-3.so.0
#52 0x00007f17165bfb3a in  () at /usr/lib/libgtk-3.so.0

----

Application: kicad
Version: (6.0.0-rc1-dev-1184-g86273e86b), debug build
Libraries:
    wxWidgets 3.0.4
    libcurl/7.62.0 OpenSSL/1.1.1 zlib/1.2.11 libidn2/2.0.5 libpsl/0.20.2
(+libidn2/2.0.4) libssh2/1.8.0 nghttp2/1.34.0
Platform: Linux 4.18.16-arch1-1-ARCH x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
    Boost: 1.68.0
    OpenCASCADE Community Edition: 6.9.1
    Curl: 7.62.0
    Compiler: GCC 8.2.1 with C++ ABI 1013

Build settings:
    USE_WX_GRAPHICS_CONTEXT=OFF
    USE_WX_OVERLAY=OFF
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_PYTHON3=ON
    KICAD_SCRIPTING_WXPYTHON=ON
    KICAD_SCRIPTING_WXPYTHON_PHOENIX=ON
    KICAD_SCRIPTING_ACTION_MENU=ON
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=ON
    KICAD_USE_OCC=OFF
    KICAD_SPICE=ON
```
Original tags: gerbview gtk3 legacy

Story points: 6.0