swscale: please support planar RGB (GBRP) output

[gjdfgh]

Planar RGB is surprisingly useful for algorithms that can work on both RGB and YUV colorspaces. (Especially if you want a cheap way to add RGB support to YUV code.) For these uses, it's really impractical that you can't use libswscale to convert input image data to GBRP.

Conversion to planar RGB should work with any input pixel format, and at least sws_isSupportedOutput(AV_PIX_FMT_GBRP) should return true.

[Carl Eugen Hoyos]

Duplicate of ticket #1757.