Fix compile warning when enable tiled backing store

Fix compile warning when enable tiled backing store
https://bugs.webkit.org/show_bug.cgi?id=95422

Patch by Kangil Han <kangil.han@samsung.com> on 2012-08-30
Reviewed by Kentaro Hara.

Fixed compile warning messages when enabled tiled backing store.
In case of TiledBackingStore, it was first thought about static_cast<unsigned>.
However, if minus value is assigned to the comparison, it would be critical.
So, it was modified as using int value in tiled coordinate calculation.

* page/Frame.cpp:
(WebCore::Frame::tiledBackingStorePaintEnd): comparison between signed and unsigned integer expressions [-Wsign-compare]
* platform/graphics/TiledBackingStore.cpp:
(WebCore::TiledBackingStore::invalidate): comparison between signed and unsigned integer expressions [-Wsign-compare]
(WebCore::TiledBackingStore::paint): comparison between signed and unsigned integer expressions [-Wsign-compare]
(WebCore::TiledBackingStore::coverageRatio): comparison between signed and unsigned integer expressions [-Wsign-compare]
(WebCore::TiledBackingStore::createTiles): comparison between signed and unsigned integer expressions [-Wsign-compare]
* platform/graphics/cairo/GLContext.cpp:
(WebCore::GLContext::createOffscreenContext): no return statement in function returning non-void [-Wreturn-type]

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@127118 268f45cc-cd09-0410-ab3c-d52691b4dbfc

Conflicts:

	Source/WebCore/platform/graphics/TiledBackingStore.cpp

Change-Id: If860f2190a7b231303c4c58e56bf0a7b9f954243