[507] Fix translucent plots of 32k/64k source into 16M
The combining shifts for 4k source are 0/4/8 <-- all even, OK 32k source are 0/5/10 64k source are 0/5/11 When the destination is a 16M colour mode (ie. full 8bpp mask) this results in an attempt to generate an odd shift of &FF which can't be encoded in a 12b immediate. The code generator asserts ("ror < 32") and thus no plot is produced.
Spot this case and stick in an extra LSL by 1 to ensure the mask can be represented, then compensate with an extra right shift during the combining ORR.