This updates glad for EGL and adds in several extensions.
EGL_EXT_platform_base
EGL_EXT_platform_xcb
EGL_KHR_image_base
EGL_KHR_image_pixmap
The xcb platform extension is unused but good to include if we want to
try removing more Xlib dependencies. platform_base is included for good
measure as other platform extensions technically build on it. image_base
and image_pixmap are for upcoming xcomposite over EGL support.
It also drops the unused create_context_no_error extensions.
Just in case glSwapIntervalEXT and glSwapIntervalSGI aren't available
for whatever reason. This entire patch is most likely completely
redundant on modern mesa drivers.
