[GPU] 3d-to-d2 texture implementation

Adds vulkan version of mode 1 and 2 and fixes related crashes by keeping
the 2d texture views from the texture cache.

src/xenia/gpu/gpu_flags.cc

@@ -89,3 +89,9 @@ DEFINE_bool(no_discard_stencil_in_transfer_pipelines, false,
             "Skip stencil bit discard in render target transfer pipelines. "
             "May improve performance on some GPUs.",
             "GPU");
+
+DEFINE_int32(gpu_3d_to_2d_texture_mode, 2,
+             "Handle shaders that sample 3D textures as 2D by creating a 2D "
+             "copy of slice 0. 0 = disabled, 1 = GPU copy, "
+             "2 = CPU re-upload from guest memory (default).",
+             "GPU");