minui: Add support for ARGB (LSB alpha)

- Called RGBA to follow convention in minui
  where big-endian description of format is used
  and to avoid conflict with existing ARGB
- Handle alpha being LSB in png and text rendering

Change-Id: I0d9afef55aad05028ed0b8b4204f1d52aaa683c3
(cherry picked from commit cd1c327427febe7cf76e6fa4835847f0d4f398e3)