Support Storage Textures as Valid Binding Types

This patch adds the basic validation of read-only storage texture,
write-only storage texture and read-write storage texture as new
binding types with no bind group layout provided in the creation of
pipeline state objects.

- Read-only storage textures can be used in vertex, fragment and
  compute shaders.
- Write-only storage textures can only be used in compute shaders
  due to the limitation on Metal.
- Read-write storage textures are not allowed now and they are
  reserved to be supported as an extension in the future.

BUG=dawn:267
TEST=dawn_unittests

Change-Id: Iffc432f29a855b85d59451cb3c50269e03b84627
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/16661
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>

src/dawn_native/opengl/CommandBufferGL.cpp

@@ -305,6 +305,8 @@ namespace dawn_native { namespace opengl {
                         } break;
 
                         case wgpu::BindingType::StorageTexture:
+                        case wgpu::BindingType::ReadonlyStorageTexture:
+                        case wgpu::BindingType::WriteonlyStorageTexture:
                             UNREACHABLE();
                             break;
 

src/dawn_native/opengl/PipelineGL.cpp

@@ -140,6 +140,8 @@ namespace dawn_native { namespace opengl {
                         break;
 
                     case wgpu::BindingType::StorageTexture:
+                    case wgpu::BindingType::ReadonlyStorageTexture:
+                    case wgpu::BindingType::WriteonlyStorageTexture:
                         UNREACHABLE();
                         break;
 

src/dawn_native/opengl/PipelineLayoutGL.cpp

@@ -56,6 +56,8 @@ namespace dawn_native { namespace opengl {
                         break;
 
                     case wgpu::BindingType::StorageTexture:
+                    case wgpu::BindingType::ReadonlyStorageTexture:
+                    case wgpu::BindingType::WriteonlyStorageTexture:
                         UNREACHABLE();
                         break;