gltestsrc: implement snow pattern with GLSL.
gltestsrc: add Mandelbrot fractal pattern.
Revert "gl: add prototypes for KHR_debug/ARB_debug_output"

This reverts commit 14a3484b14a3d4312dea6e107c6f29941176cde9.
Revert "glcontext: implement the debug callback"

This reverts commit 95a4585d4c554aae5b1ee057dc7ba701c848a82c.