Minor: Added a `srcBytes != NULL` assert

• Added an assert to cgTextureMappingBlit() to make sure that the srcBytes returned from `CFDataGetBytePtr()` isn’t NULL.
	» It would crash later if this were the case; this makes sure we crash early, on the core issue.
Added GLKMathExtensions & MathExtensions to copied includes

• Added `GLKMathExtensions.h`/`.inl` and `MathExtensions.h``.inl` to Copy Files phase of libCGTextureMapping.
	» Allows inclusion and usage of these by encompassing projects.