Remove xform canvas mode from Viewer, simplify color management

There are now three modes: Legacy, and Color Managed 8888 or F16. Rules
about transfer functions are gone, so allow changing gamma in either
color managed mode.

To keep things much simpler, we always construct the window surface in
legacy mode, and draw offscreen whenever we're doing any color
management. This lets us avoid re-creating the window surface when
cycling modes, and avoid making re-tagged copies when pushing color
managed images to the window surface.

Bug: skia:
Change-Id: I59e9340900b047d5217eb8a9f63f20a1d638227d
Reviewed-on: https://skia-review.googlesource.com/c/172960
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
diff --git a/tools/viewer/Viewer.h b/tools/viewer/Viewer.h
index 02e4063..f2fd3e7 100644
--- a/tools/viewer/Viewer.h
+++ b/tools/viewer/Viewer.h
@@ -80,10 +80,9 @@
     };
 private:
     enum class ColorMode {
-        kLegacy,                                 // N32, no color management
-        kColorManagedSRGB8888_NonLinearBlending, // N32, sRGB transfer function, nonlinear blending
-        kColorManagedSRGB8888,                   // N32, sRGB transfer function, linear blending
-        kColorManagedLinearF16,                  // F16, linear transfer function, linear blending
+        kLegacy,            // 8888, no color management
+        kColorManaged8888,  // 8888 with color management
+        kColorManagedF16,   // F16 with color management
     };
 
     void initSlides();