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.
Georges Basile Stavracas Neto
Kurt Kartaltepe
PatTheMav
Richard Stanway
jp9000
Matt Gajownik
Ryan Foster
Colin Edwards
fryshorts
martell
BtbN