Added support for DXT1/3/5 and BC5/7 compressed textures via love.image.CompressedData. Some internals are still iffy.

- added a new CompressedData type to love.image
- added love.image.isCompressed(file or data)
- love.graphics.newImage automatically tests for compression when loading a file
- added love.graphics.isSupported strings for DXT, BC5, and BC7

Compressed textures meant to be used on the GPU offer huge performance gains when
- loading the texture from a file
- loading the texture from RAM to VRAM
- loading mipmaps
- rendering

--HG--
branch : image-CompressedData

src/modules/graphics/Graphics.cpp

@@ -163,6 +163,9 @@ StringMap<Graphics::Support, Graphics::SUPPORT_MAX_ENUM>::Entry Graphics::suppor
 	{ "npot", Graphics::SUPPORT_NPOT },
 	{ "subtractive", Graphics::SUPPORT_SUBTRACTIVE },
 	{ "mipmap", Graphics::SUPPORT_MIPMAP },
+	{ "dxt", Graphics::SUPPORT_DXT },
+	{ "bc5", Graphics::SUPPORT_BC5 },
+	{ "bc7", Graphics::SUPPORT_BC7 },
 };
 
 StringMap<Graphics::Support, Graphics::SUPPORT_MAX_ENUM> Graphics::support(Graphics::supportEntries, sizeof(Graphics::supportEntries));