EXR: more standard RGBA channel name detection

Various EXR images especially from movie assets do not simply have "R"
etc as channel names; the convention seems to be to have "layer name",
".", "color channel name". For example, channels in EXR might be
"rgb.R", "rgb.G", "rgb.B", which RenderDoc was not detecting previously
and displaying an all-black image.

renderdoc/core/image_viewer.cpp

@@ -758,10 +758,14 @@ void ImageViewer::RefreshFile()
       return;
     }
 
+    // Expect R/G/B/A channel names as first char of the string, or
+    // after the last '.' char.
     int channels[4] = {-1, -1, -1, -1};
     for(int i = 0; i < exrImage.num_channels; i++)
     {
-      switch(exrHeader.channels[i].name[0])
+      const char *dotPos = strrchr(exrHeader.channels[i].name, '.');
+      const char *name = dotPos ? dotPos + 1 : exrHeader.channels[i].name;
+      switch(name[0])
       {
         case 'R': channels[0] = i; break;
         case 'G': channels[1] = i; break;