CopyTexture should force mipmap generation for src if it is RT with (auto)generateMips set to true

A RT may not have generated its mipmaps prior to being used as the src in a CopyTexture call. It would be nicer for users if CopyTexture made sure the src RT has generated its mipmaps before trying to copy it.

I've attached a project that can be used to verify this change. The Blit is done on the RT before the CopyTexture and currently the CopyTexture doens't copy the mip levels > 0 as the mipmaps weren't generated yet.

To repro:
1: Open attached project
2: Run in Editor on Windows OpenGLCore (OpenGL 4.5)
3: See that the bottom left quad is black when mip level is >0 (distance is futher)
4: After forcing mipmap generation in CopyTexture that bottom left quad should have all mipmap levels