Combine 'special' with 'specialFormat' as single resource format type

* There was no good reason to have a flag indicating if the special
  format was valid or not. Now it's a single enum, with a value
  'Regular' indicating that the compCount/compWidth/compType fully
  describe the format itself.
* This makes code patterns easier as you no longer need to check for
  special then check for specialFormat, you can just test the type
  directly.

renderdoc/core/image_viewer.cpp

@@ -418,7 +418,7 @@ void ImageViewer::RefreshFile()
   rgba8_unorm.compByteWidth = 1;
   rgba8_unorm.compCount = 4;
   rgba8_unorm.compType = CompType::UNorm;
-  rgba8_unorm.special = false;
+  rgba8_unorm.type = ResourceFormatType::Regular;
 
   ResourceFormat rgba32_float = rgba8_unorm;
   rgba32_float.compByteWidth = 4;