[naga] Implement ToWgsl for ImageDimension, and use as needed.

Replace `naga::back::wgsl::writer::image_dimension_str` with a
`ToWgsl` implementation for `ImageDimension` in `common::wgsl`. Use
this where needed in the WGSL backend.

Author: jimblandy

naga/src/back/wgsl/writer.rs

@@ -470,7 +470,7 @@ impl<W: Write> Writer<W> {
                 // More about texture types: https://gpuweb.github.io/gpuweb/wgsl/#sampled-texture-type
                 use crate::ImageClass as Ic;
 
-                let dim_str = image_dimension_str(dim);
+                let dim_str = dim.to_wgsl_if_implemented()?;
                 let arrayed_str = if arrayed { "_array" } else { "" };
                 let (class_str, multisampled_str, format_str, storage_str) = match class {
                     Ic::Sampled { kind, multi } => (
@@ -1904,17 +1904,6 @@ impl<W: Write> Writer<W> {
     }
 }
 
-const fn image_dimension_str(dim: crate::ImageDimension) -> &'static str {
-    use crate::ImageDimension as IDim;
-
-    match dim {
-        IDim::D1 => "1d",
-        IDim::D2 => "2d",
-        IDim::D3 => "3d",
-        IDim::Cube => "cube",
-    }
-}
-
 const fn address_space_str(
     space: crate::AddressSpace,
 ) -> (Option<&'static str>, Option<&'static str>) {

naga/src/common/wgsl.rs

@@ -306,3 +306,18 @@ impl ToWgsl for crate::Scalar {
         })
     }
 }
+
+impl ToWgsl for crate::ImageDimension {
+    const DESCRIPTION: &'static str = "image dimension";
+
+    fn to_wgsl(self) -> Option<&'static str> {
+        use crate::ImageDimension as IDim;
+
+        Some(match self {
+            IDim::D1 => "1d",
+            IDim::D2 => "2d",
+            IDim::D3 => "3d",
+            IDim::Cube => "cube",
+        })
+    }
+}