Add definitions for iOS / GLES2 extension functions and constants

[notlion]

This fixes the iOS build when compiling with -DCINDER_GL_ES_2 on Xcode 13.3.

Some of these were already defined in RendererImplGlCocoaTouch.mm. I've moved those definitions to platform.h since some are needed in other files. There's still one instance of an glResolveMultisampleFramebufferAPPLE being used. I've left this one unchanged since it's in a platform specific file.