ImageViewer get the Custom Texture Description from the proxy

Use the cached Texture Description for all other Textures which matches the behaviour of GetTextureData()

When saving a texture when a custom visualisation shader is in use:
- fixes a crash when saving as HDR or EXT
- fixes incorrect image saved to DDS

renderdoc/core/image_viewer.cpp

@@ -206,7 +206,12 @@ public:
   }
   rdcarray<ResourceDescription> GetResources() { return m_Resources; }
   rdcarray<TextureDescription> GetTextures() { return {m_TexDetails}; }
-  TextureDescription GetTexture(ResourceId id) { return m_TexDetails; }
+  TextureDescription GetTexture(ResourceId id)
+  {
+    if(id != m_CustomTexID)
+      return m_TexDetails;
+    return m_Proxy->GetTexture(id);
+  }
   void GetTextureData(ResourceId tex, const Subresource &sub, const GetTextureDataParams &params,
                       bytebuf &data)
   {