[FIXED] expose CL_APPLE_gl_sharing via CLPlatformCapabilities

[Matthias]

CLPlatformCapabilities is missing fields for common apple extensions like "CL_APPLE_gl_sharing".
Apple doesn't expose "CL_KHR_gl_sharing" so one needs to check for the apple version too, which currently requires to manually parse the extension string.

[spasi]

I've added 3 APPLE extensions that are exposed in the CL_PLATFORM_EXTENSIONS string to CLPlatformCapabilities. Keep in mind though that you also need to check the CLDeviceCapabilities for each of your devices before using a CL extension (a flag for APPLE_gl_sharing was already present there).