minui: Move GRSurface into a class.

This CL adds GRSurface::Create() and dtor for managing the allocated
memory in GRSurface class. It also adds GRSurface::data() that hides the
underlying implementation, with both of const and non-const overloads.
This allows `const GRSurface&` to be more useful - previously it only
ensured a const member variable of `data`, instead of a read-only buffer
it points to.

It also marks the parameters in gr_texticon() and gr_blit() as const, as
they're incoming source that shouldn't be altered. It corrects the type
of gr_draw, which is the sink to be painted on (an earlier attempt was
made in [1], but didn't get the full picture correctly).

[1] https://android-review.googlesource.com/c/platform/bootable/recovery/+/704757/

Test: mmma -j bootable/recovery
Test: recovery_unit_test on marlin
Test: Run graphics test on marlin (fbdev).
Test: Run graphics test on blueline (drm).
Change-Id: I7904df084cd6c08fa04a9da97d01b4b1a6e3a20c