Trac #19356
Issue=273
Signed-off-by: Nicolas Capens
Based on preliminary patch from Ken Russell

Images are never renderable directly, so renaming the Image::isRenderable function clarifies that we are
only checking on the renderability of the format.
Also factor out the lookup to a static helper function (it will be used elsewhere in the next patch)

git-svn-id: https://angleproject.googlecode.com/svn/trunk@926 736b8ea6-26fd-11df-bfd4-992fa37f6226