Gif: zero filling should use memset instead of setRGBA for every pixel

Gif: zero filling should use memset instead of setRGBA for every pixel
https://bugs.webkit.org/show_bug.cgi?id=118350

Patch by Balazs Kelemen <b.kelemen@samsung.com> on 2013-07-03
Reviewed by Allan Sandfeld Jensen.

No new tests. Actually it is not covered by existing tests. Surprisingly we haven't got pixel
tests for animated images. Given that this patch is pretty trivial I don't think it's worth the
cost to start introducing such tests.
I added a manual test: animated-gif-dispose-background.html.

GIFImageDecoder::initializeFrameBuffer use a loop to fill a subrect with tranparent pixels.
This is extremely ineffecient. The use case for this code path is not frequent on the web
but it's still better to fix it.

* platform/image-decoders/ImageDecoder.cpp:
(WebCore::ImageFrame::zeroFillFrameRect):
* platform/image-decoders/ImageDecoder.h:
* platform/image-decoders/gif/GIFImageDecoder.cpp:
(WebCore::GIFImageDecoder::initFrameBuffer):
Fixed indentation in addition.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@152352 268f45cc-cd09-0410-ab3c-d52691b4dbfc