Cycles-Bake: UI changes: removing External Saving options

Campbell and I discussed and we decided to deliver only the basic feature-set before moving into new additions.
That doesn't mean cage will be removed, but let's reevaluate the need for saving externally after bake-cycles 1.0 gets in master.

If you saved a file with the branch you should do this in the python console:
bpy.context.scene.render.bake.save_mode = 'INTERNAL'
IMB buffer functions to take a mask as input

IMB_buffer_byte_from_float_mask and IMB_buffer_float_from_float_mask
The mask make sure the conversion only happens in a few areas of the
buffer.
Cycles-Bake: "Clear" option fully supported
build fix for windows after 68893054 (IMB buffer functions to take a mask as input)

Actually ... this was a left over from debugging, so Im glad it was spotted
Cycles-Bake: [review] Cycles style cleanup
Revert "Cycles-Bake: [review] using MACRO in get_shader_type()"

This reverts commit 1f8c36d02c2b62387c2bb7c3f29ddb722771b2de.