Basic GLX_EXT_texture_from_pixmap support in ApiTrace

Apitrace now has basic support for the GLX_EXT_texture_from_pixmap extension, frequently used by Linux desktop compositors.

The difficulty with this extension was that it basically allows sharing textures between different processes, but when replaying those external processes and textures are not available. The solution was to emit fake glTexImage2D calls with the contents of the shared textures whenever they are used. (The same technique was already being for tracing OpenGL's GL_OES_EGL_image extension, and Direct3D's shared resources.)

This does yield large traces which are inadequate for profiling (something that can be eventually addressed with more effort.) But replays should be now visually faithful, therefore useful for debugging correctness/rendering issues.