Requires addition of viewports and alternate framebuffers to greylib
which are essentially similar to the core implementation except that
the framebuffer can be any size and relationship to a viewport. Drawing
is always fully clipped to the intersecting area.
Adapt oscilloscope.c to the API change. FFT will use the new features
(later update).
Get rid of silly and wrong lcd_bmp_part use in OSD. Remove it from
plugin API (must be made incompatible now).
Change-Id: Iafa5e2174148fb8ad11db6b9d4add0dcabc5c563
