Full support for DXT/S3TC textures
Based on reported extensions for OpenGL, DXT mipmapped textures are now supported by all devices that claim support for the formats.
Merge branch 'develop' of https://github.com/mono/MonoGame into full-dxt-support
Merge branch 'develop' of https://github.com/mono/MonoGame into full-dxt-support

Conflicts:
	MonoGame.Framework/Graphics/GraphicsExtensions.cs
Merge branch 'develop' of https://github.com/mono/MonoGame into full-dxt-support

Conflicts:
	MonoGame.Framework/Graphics/Texture.cs
Corrected case for DxtBitmapContent, PvrtcBitmapContent.
DxtBitmapContent now matches XNA and C# naming conventions.
PvrtcBitmapContent follows suit.
Second part of rename.
Fix missing abstract keyword.
Fix reference to renamed PvrtcBitmapContent class.
Merge branch 'develop' of https://github.com/mono/MonoGame into full-dxt-support

Conflicts:
	MonoGame.Framework/Graphics/GraphicsExtensions.cs
	MonoGame.Framework/Graphics/Texture2D.cs
	MonoGame.Framework/Graphics/TextureCube.cs
Merge pull request #1662 from slygamer/full-dxt-support

Full support for DXT/S3TC textures
Merge back from upstream, resolving conflicts in Texture2D... again...