Bug 934787 - Improve the WebGL code of the Gallery app

Bug 934790 - Gallery's WebGL code should not call texImage2D on every draw - r=jgilbert,djf

Bug 934813 - Gallery's WebGL code should not upload buffers on every draw - r=jgilbert,djf

Bug 935353 - Gallery's WebGL code: avoid divisions in the fragment shader, and don't use a matrix to store separate vectors - r=djf,jgilbert

Modify one of Benoit's patches to change the flag used for uploading the preview image and to fix lint r=dmarcos

Allow 4 times as many stops on the exposure slider r=dmarcos

Use requestAnimationFrame() for the exposure changes
Merge pull request #14520 from davidflanagan/bug934787

Bug 934787 - Improve the WebGL code of the Gallery app r=jgilbert,djf,dmarcos