CogSource: handle JPG/YCbCr with transparency mask

ℹ️ Funded by an industrial customer.

Currently, geotiff.js does not support COG files encoded in JPEG and YCbCr color space and an additional transparency mask. OpenLayers does support it by doing a bit of magic with geotiff.js : first the regular RGB buffer is read with readRGB(), then the mask is read with readRaster(), and then both buffers are combined to have the final RGBA buffer.