VBA-M

The attached patch fixes several glitches when changing display settings in
the GTK UI.

The current OpenGL screen area does not update the texture sizes when
filters are changed in the display settings (but the underlying pixel
buffer size changes). That causes several problems when changing display
settings on-the-fly. Here's one way to reproduce the problem consistently
(there are several ways, actually):

1) Launch gvbam, disable all 2x display filters.
2) Quit gvbam.
3) Launch gvbam and load a ROM.
4) Try to select a display filter.
5) Screen will turn into garbage and/or freeze, but emulation will
otherwise continue just fine.

This happens because the texture size specified on on_realize when using no
filters is X * Y, and when you use a filter it becomes 2 * X * 2 * Y
(that's what you'll try to map anyways). The attached patch monitors those
changes to specify the correct texture size at all times, correcting the
issue.

I tried to adhere to the coding style as much as possible, but I'm not used
to this coding style so I might have made a few mistakes, please let me
know. The patch is in regular unified diff format and not in SVN diff
format because I don't like working with SVN (I use a git-svn bridge), but
I can provide a patch in svn diff format if needed, no problem.