Remove global g_xres and x_yres

Instead query g_VideoMode where needed. As both g_xres and g_yres aren't
global anymore remove static from UpdateRenderer. While at it use more
desicriptive names: GetXRes -> GetWindowWidth, GetYRes ->
GetWindowHeight.

This reduces the amount of variables tracking width and height by one
each.

Signed-off-by: Ralph Sennhauser <ralph.sennhauser@gmail.com>

source/ps/GameSetup/Config.cpp

@@ -30,8 +30,6 @@
 // (these variables are documented in the header.)
 bool g_PauseOnFocusLoss = false;
 
-int g_xres, g_yres;
-
 bool g_Quickstart = false;
 bool g_DisableAudio = false;