[rendercapture] Fix passthrough rendercapture interface

The RenderCapture function doesn't behave correctly for passthough video renderers,
and in fact segfaults on Pi when setting a bookmark.

There is no need to do a glReadPixels to get a video snapshot in the passthrough case,
so provide a shortcut.
[rbp] Support grabbing just the video layer through rendercapture

As the bookmark snapshot doesn't want the OSD (and neither does boblight)
allow video, or video + GUI to be selectable.

While we're there, making the GPU do the red/blue swap and stride packing saves some cpu.

Requires updated firmware
Merge pull request #3210 from popcornmix/passthrough_capture

[rendercapture] Fix passthrough rendercapture interface