refactor image resource loading code in minui

Reduce the number of copies of libpng boilerplate.  Rename
res_create_* functions to be more clear.  Make explicit the use of the
framebuffer pixel format for images, and handle more combinations of
input and output (eg, loading a grayscale image for display rather
than use as a text alpha channel).

Change-Id: I3d41c800a8f4c22b2f0167967ce6ee4d6b2b8846