Enable basic support for 'RGBa' raw encoding/decoding

[immerrr]

This enables creating images in RGBa mode with frombytes or frombuffer directly, previously raw_decoder and raw_encoder would choke even on RGBa -> RGBa no-op transformations.

This also enables band-shuffling codecs that were readily available for RGBA to be used for RGBa. One motivation for that is to speed-up conversion to and from QImage in QImage.Format_ARGB32_Premultiplied.

[landscape-bot]

Repository health increased by 0.94% when pulling d41fed6 on immerrr:rgba-premultiplied-codecs into 5fa52f8 on python-pillow:master.

• 47 new problems were found (including 1 error and 45 code smells).
• 75 problems were fixed (including 37 errors and 20 code smells).

[wiredfool]

Looks good to me, thanks for the patch.