Add a GrCustomCoordsTextureEffect class

Add a GrCustomCoordsTextureEffect class

Extracts a GrCustomCoordsTextureEffect class from
GrSimpleTextureEffect. This way there are no effects that can
conditionally require a vertex shader. They either always need one or
never do. Also removes kCustom_CoordsType from the CoordsType enum in
GrEffect (that enum is really only meant for coords provided by the
framework), and updates GrSimpleTextureEffect::TestCreate to make the
effect with position as well, instead of just local coords.

R=bsalomon@google.com

Author: cdalton@nvidia.com

Review URL: https://codereview.chromium.org/24018007

git-svn-id: http://skia.googlecode.com/svn/trunk@11531 2bbb7eff-a529-9590-31e7-b0007b416f81