Introduces new SingleTextureEffect base class for GrCustomStage objects.

Introduces new SingleTextureEffect base class for GrCustomStage objects.
This class tracks the texture that the object uses. A future commit will get rid of the
GrTexture pointer currenty stored in the GrDrawState, allowing us to have CustomStages
*without* textures.

Requires gyp change on next roll.

http://codereview.appspot.com/6306097/

git-svn-id: http://skia.googlecode.com/svn/trunk@4576 2bbb7eff-a529-9590-31e7-b0007b416f81