app: add new operation gimp:cast-format

which casts between two arbitrary formats of the same bpp.
app: add gimp_image_set_gamma_hack()

Which when enabled casts between linear and gamma-corrected data at
the operation's input and output. This is experimental stuff meant to
properly check ported gegl ops and prepare for plug-in removal.
app: add a "Gamma hack" toggle to the GimpImageMap dialog