texture_cache: Implement rendering to 3D textures

This allows rendering to 3D textures with more than one slice.
Applications are allowed to render to more than one slice of a texture
using gl_Layer from a VTG shader.

This also requires reworking how 3D texture collisions are handled, for
now, this commit allows rendering to slices but not to miplevels. When a
render target attempts to write to a mipmap, we fallback to the previous
implementation (copying or flushing as needed).

- Fixes color correction 3D textures on UE4 games (rainbow effects).
- Allows Xenoblade games to render to 3D textures directly.

src/video_core/engines/maxwell_3d.h

@@ -598,6 +598,7 @@ public:
                 BitField<4, 3, u32> block_height;
                 BitField<8, 3, u32> block_depth;
                 BitField<12, 1, InvMemoryLayout> type;
+                BitField<16, 1, u32> is_3d;
             } memory_layout;
             union {
                 BitField<0, 16, u32> layers;