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.
Some CMake checks were recently switched from UNIX to LINUX to get them
to not apply to macOS/OSX. Since LINUX doesn't seem to be defined,
switch these checks to UNIX AND NOT APPLE.
Just in case glSwapIntervalEXT and glSwapIntervalSGI aren't available
for whatever reason. This entire patch is most likely completely
redundant on modern mesa drivers.
