Support different formats in the same resource pool.

cc/resources/resource_format.h

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_RESOURCES_RESOURCE_FORMAT_H_
 #define CC_RESOURCES_RESOURCE_FORMAT_H_
 
 #include "base/logging.h"
 #include "third_party/skia/include/core/SkBitmap.h"
 
 namespace cc {
 
 // Keep in sync with arrays below.
 enum ResourceFormat {
   RGBA_8888,
   RGBA_4444,
   BGRA_8888,
   ALPHA_8,
   LUMINANCE_8,
   RGB_565,
   ETC1,
   RED_8,
   RESOURCE_FORMAT_MAX = RED_8,
 };
 
 SkColorType ResourceFormatToSkColorType(ResourceFormat format);
 
+bool IsFormatCompressed(ResourceFormat format);
+
+enum ResourceFormatUsage {

[reveman, 2015/01/13 16:25:55]

Do we need this? I found it a bit hard to follow how this maps to a format. 

[peterp, 2015/01/14 12:33:36]

NATIVE will generally map to RGBA_4444, RGBA_8888 or BGRA_8888.
TRANSLUSCENT will generally map to ATC_IA, DXT5 or any of the NATIVE formats.
OPAQUE will generally map to ATC, DXT1, ETC1 or any of the NATIVE formats.

NATIVE is there because no GPU that I know of is capable of rendering directly
to a compressed format. Since there's no encoding it's also the fastest format
to use when generating tiles (but usually slower when OpenGL composits the final
fragments to the destination surface).

TRANSLUSCENT will generally achieve a 4:1 compression ratio and is the slowest
to encode but sometimes we need the alpha channel for blending in the
compositor.

OPAQUE will generally achive a 8:1 compression ratio and is the fastest to
encode. In practice we've found that this will be most of the tiles.

Maybe I should put all this in a comment for the enum?

+  // Fastest format. Also supported for GPU rasterization.
+  FORMAT_USAGE_NATIVE,
+  // Memory efficient format that also supports alpha channel.
+  FORMAT_USAGE_TRANSLUSCENT,
+  // Memory efficient format that only supports color channels.
+  FORMAT_USAGE_OPAQUE,
+  NUM_RESOURCE_FORMAT_USAGE
+};
+
 }  // namespace cc
 
 #endif  // CC_RESOURCES_RESOURCE_FORMAT_H_