khronos: Update headers.
specs: Support GL_ARB_direct_state_access.
specs: Support GL_ARB_get_texture_sub_image.
specs: Support GL_KHR_robustness.
specs: Reorder GL_*_IBM params numerically.
specs: Update GL enums.
specs: Add missing 4.5 extensions.

GL_ARB_clip_control and GL_ARB_texture_barrier
specs: Remove redundant _glTexImage*_size variants.