Allow texture-backed bitmaps to perform a read-back when lockPixels is called.

Allow texture-backed bitmaps to perform a read-back when lockPixels is called.
This means we have to be even more cautious about when we call lock, and we should
always check getTexture() first if we can handle a texture directly, rather than
forcing the read-back to get the bits.

git-svn-id: http://skia.googlecode.com/svn/trunk@1815 2bbb7eff-a529-9590-31e7-b0007b416f81