Win32: basic initialization

Initialization of Win32 with WGL and EGL.

Move variable declarations to beginning of block. Use alloca instead of
dynamically sized arrays. Compiles with MSVC.
Win32 event handling
Win32: special keys and mouse wheel

Handle WM_SYSKEYUP, WM_SYSKEYDOWN, translate keysyms to distinguish
between left and right ctrl, alt, etc.

WM_MOUSEWHEEL, WM_MOUSEHWHEEL send button events.
EGL, WGL, GLX: fixes

Fix glwtMakeCurrent(NULL).

Add error status return values to glwtMakeCurrent, glwtSwapBuffers,
glwtSwapInterval and glwtWindowGetSize.

Get rid of platform specific #ifdefs in EGL code. Move
visual_id/pixel_format from x11, win32 to glx, wgl, egl.

Split glwtWindowCreateEGL context and surface creation.
X11, Win32: use switch-case for keymap

Using a look-up table was not such a good idea after all.
GLX, WGL: OpenGL ES contexts

Pass OpenGL ES context flags when creating contexts with GLX and WGL.

Link tests with GLXW and clear the screen with OpenGL.
Fixed rpi compilation with egl changes

rpi/window.c passed the native dispmanx window
by value instead of pointer but according to
eglplatform.h EGLNativeWindowType is void*.
Moved type enum out of GLWTWindowEvent

this helps keeping syntax consistent between C and
C++ otherwise all uses of the enum constants
have to be preposed by GLWTWindowEvent::
Update GLXW submodule