rg_etc1: Fix encoding of solid color blocks

rg_etc1: Fix encoding of solid color blocks

The ETC1 encoder is expected to write all 8 bytes of the
output data. But in case of a solid color block, it was writing
only 1 of the first 3 bytes (R, G, B). So lots of solid blocks
were containing invalid data (for instance: R + dR < 0 or > 255).