[hgiMetal] Change Metal's swizzle so single channel texture

reads follow the UsdPreviewSurface spec of repeating the single channel
on the r, g and b channels with 1.0 in the alpha channel.

Fixes #2731

(Internal change: 2308645)

pxr/imaging/hgiMetal/texture.mm

@@ -74,24 +74,15 @@
 #if (defined(__MAC_10_15) && __MAC_OS_X_VERSION_MAX_ALLOWED >= __MAC_10_15) \
     || __IPHONE_OS_VERSION_MAX_ALLOWED >= 130000
         if (@available(macOS 10.15, ios 13.0, *)) {
-            size_t numChannels = HgiGetComponentCount(desc.format);
-
-            if (usage == MTLTextureUsageShaderRead && numChannels == 1) {
-                MTLTextureSwizzle s = HgiMetalConversions::GetComponentSwizzle(
-                    desc.componentMapping.r);
-                texDesc.swizzle = MTLTextureSwizzleChannelsMake(s, s, s, s);
-            }
-            else {
-                texDesc.swizzle = MTLTextureSwizzleChannelsMake(
-                    HgiMetalConversions::GetComponentSwizzle(
-                        desc.componentMapping.r),
-                    HgiMetalConversions::GetComponentSwizzle(
-                        desc.componentMapping.g),
-                    HgiMetalConversions::GetComponentSwizzle(
-                        desc.componentMapping.b),
-                    HgiMetalConversions::GetComponentSwizzle(
-                        desc.componentMapping.a));
-            }
+            texDesc.swizzle = MTLTextureSwizzleChannelsMake(
+                HgiMetalConversions::GetComponentSwizzle(
+                    desc.componentMapping.r),
+                HgiMetalConversions::GetComponentSwizzle(
+                    desc.componentMapping.g),
+                HgiMetalConversions::GetComponentSwizzle(
+                    desc.componentMapping.b),
+                HgiMetalConversions::GetComponentSwizzle(
+                    desc.componentMapping.a));
         }
 #endif