remove most platform dependent exceptions for SDL2 code

* switch travis to trusty
* new parameter fullscreendesktop: uses desktop resolution for fullscreen
* "colorbits" removed, since it's no longer supported
* scr_w/scr_h no longer necessarily reflect the true display resolution
  (with fullscreendesktop = 1 and fullscreen = 1, scr_w and scr_h are
  ignored)
* desktopw and desktoph are now guaranteed to have nonzero values
* fallback to old mouse behaviour when SDL_SetRelativeMouseMode fails
* require a click into the window to grab the mouse
* scr_w/scr_h default to desktop resolution on first run

thx sauerbraten :)

DOCREF fullscreendesktop colorbits

source/src/rendergl.cpp

@@ -125,7 +125,7 @@ void gl_checkextensions()
 #endif
 }
 
-void gl_init(int w, int h, int bpp, int depth, int fsaa)
+void gl_init(int w, int h, int depth, int fsaa)
 {
     //#define fogvalues 0.5f, 0.6f, 0.7f, 1.0f