GPUImagePicture: don't redraw using CoreGraphics unless you have to

There was support for this but it appeared to have been disabled.
GPUImagePicture: only access image bytes directly if the bitmap byte order and format is compatible with GL

This correct non-redrawing for JPGs from my previous commit, as they are big endian.
Merge pull request #1267 from karlvr/gpuimagepicture-redraw

GPUImagePicture: don't redraw using CoreGraphics unless you have to