Make SkImage::getTexture() const. At the moment the function
does not mutate the state.
One use-case is that this makes it possible in the future to add draw
function to SkBaseDevice functions. The device draw functions take
const ref objects, but SkGpuDevice likely would benefit of using the
getTexture().
BUG=skia:3388
R=reed@google.com
Review URL: https://codereview.chromium.org/925853002
cd6ca9e689