Rendertarget attachments, texture and image changes (#358)

* Add multiple color outputs for fragment shaders

* Add registers and gal enums

* Use textures for framebuffers and split color and zeta framebuffers

* Abstract texture and framebuffer targets as an image

* Share images between framebuffers and textures

* Unstub formats

* Add some formats

* Disable multiple attachments

* Cache framebuffer attachments

* Handle format types

* Add some rendertarget formats

* Code cleanup

* Fixup half float types

* Address feedback

* Disable multiple attachments in shaders

* Add A4B4G4R4 image format

* Add reversed section for image enums

Ryujinx.Graphics/Gal/GalTextureFormat.cs

@@ -9,6 +9,7 @@ namespace Ryujinx.Graphics.Gal
         R32          = 0xf,
         BC6H_SF16    = 0x10,
         BC6H_UF16    = 0x11,
+        A4B4G4R4     = 0x12,
         A1B5G5R5     = 0x14,
         B5G6R5       = 0x15,
         BC7U         = 0x17,