Fixed bug 1837 - Use error extension instead of glGetError()

Implemented support for GL_ARB_debug_output, but was unable to test it on Mac OS X.
Fixed bug 1842 - [patch] SDL_SetWindowPosition sets bad position values when given SDL_WINDOWPOS_CENTERED args

Alex Szpakowski

When calling SDL_SetWindowPosition(window, SDL_WINDOWPOS_CENTERED, SDL_WINDOWPOS_CENTERED), the window moves to the correct position but it seems to internally set its x/y position values to the literal value of the SDL_WINDOWPOS_CENTERED define.
This causes all sorts of problems when SDL functions which use the window position (e.g. SDL_SetWindowGrab) are called after the aforementioned SDL_SetWindowPosition call.

Looking at the SDL_SetWindowPosition code, it seems that SDL_SendWindowEvent with the SDL_WINDOWEVENT_MOVED event is called at the end of the function using the literal value of the SDL_WINDOWPOS_CENTERED define, instead of the newly set window->x and window->y values.
SDL_SendWindowEvent then sets the values of window->windowed.x and window->windowed.y to that value (0x2FFF0000, aka 805240832.)

I have attached a patch which changes SDL_SetWindowPosition to make sure SDL_SendWindowEvent is called with the correct coordinate values, if SDL_WINDOWPOS_CENTERED is used (fixes the issue for me.)

Tested with Mac OS 10.8.3.
Fixed windows build
Fixed declaration of GL_HandleDebugMessage