shader/image: Implement SULD and remove irrelevant code

* Implement SULD as float.
* Remove conditional declaration of GL_ARB_shader_viewport_layer_array.

src/video_core/engines/shader_bytecode.h

@@ -1427,7 +1427,7 @@ union Instruction {
             ASSERT(mode == SurfaceDataMode::D_BA);
             return store_data_layout;
         }
-    } sust;
+    } suldst;
 
     union {
         BitField<28, 1, u64> is_ba;