Add support for multisampled rendertargets in most OpenGL and OpenGLES platforms
Fix mac build
Disable msaa support for MonoMac (for now)
Fixed GL.RenderbufferStorageMultisample not supported in MonoMac
Add PlatformResolveRenderTargets placeholder to all platforms
Fix Monomac msaa support
Merge branch 'develop' of https://github.com/mono/MonoGame into opengl_msaa

Conflicts:
	MonoGame.Framework/Graphics/RenderTarget2D.OpenGL.cs
Merge pull request #2782 from flyingdevelopmentstudio/opengl_msaa

Add support for multisampled rendertargets in most OpenGL platforms
Merge back from develop
Pull GL-specific rectangle flips into GLDevice.