widget: Make background tint colour use GdkRGBA

Remove vte_terminal_set_background_tint_color and vte_terminal_set_opacity.
The opacity is now taken from the alpha component of the tint GdkRGBA.

Conflicts:
	doc/reference/vte-sections.txt
	src/vte.c
	src/vte.h
	src/vteapp.c
widget: Store background saturation as double

Conflicts:
	src/vte-private.h
vteapp: Use RGBA colour APIs

Conflicts:
	src/vteapp.c
widget: Remove public GdkColor APIs

Use GdkRGBA in the API, and use PangoColor internally, and for the attributes.
widget: Remove faux-transparency support

Remove snapshotting the X root window used to emulate transparency on
non-composited WMs. There also goes the last explicit X dependency.
widget: Remove pixmap background saturation setting
widget: Remove background pixbuf settings
widget: Handle background colour changes immediately

Now that we don't support faux transparency anymore, we can just handle
background updates immediately (resulting in an invalidate-all) instead of
queuing them.
widget: Use gobject define instead of redefining it
widget: Merge paint function into ::draw

No need for a separate function here.
widget: Remove expose event debug output

It's just ::draw now, no need for two separate debug messages for the same thing.
widget: Use allocated {width,height}
drawing: Remove start/end

_vte_draw_start/end doesn't fit with the current drawing model anymore.
Instead just set the cairo context at the start of ::draw, and unset it
at the end.
drawing: Don't store the widget

And esp. not a reference to it! It's just needed when creating the font info,
so only pass it to set_text_font.
widget: No need to clip the cairo context

The context passed to ::draw is already clipped.
vteapp: Implement --border-width option

This is to test the condition that the terminal isn't at (0,0) in its
containing widget.

Conflicts:
	src/vteapp.c
widget: Remove unnecessary boolean return value

It's not called from a timeout anymore, so just return void.
widget: Remove use of deprecated GtkStyle
all: Don't export _vte_debug_* symbols

Debug symbols are for internal use only.

Conflicts:
	src/Makefile.am
widget: Remove deprecated function