spirv: Add SpvCapabilityShaderViewportIndexLayerEXT

This capability allows gl_ViewportIndex and gl_Layer to also be used
as outputs in Vertex and Tesselation shaders.

v2: Make conditional to the capability, add gl_Layer, add tesselation
    shaders. (Iago)

v3: Don't export to tesselation control shader.

v4: Add Reviewd-by tag.

Reviewed-by: Iago Toral Quiroga <itoral@igalia.com>

src/compiler/spirv/spirv_to_nir.c

@@ -3203,6 +3203,10 @@ vtn_handle_preamble_instruction(struct vtn_builder *b, SpvOp opcode,
          spv_check_supported(storage_16bit, cap);
          break;
 
+      case SpvCapabilityShaderViewportIndexLayerEXT:
+         spv_check_supported(shader_viewport_index_layer, cap);
+         break;
+
       default:
          vtn_fail("Unhandled capability");
       }