Bug 699769: Fix bugs in upsampling code for JPX images.

The upsampling code in the JPX decode attempted to guess a
suitable upsampling factor. The guessed factor was wrong,
causing writes of samples outside of the decoded image buffer.

Simply limiting the coordinates to the image buffer would
not suffice because the factor was wrong for every upsampled
row of pixels. openjpeg does provide an upsampling factor,
so use that instead and also take the component offsets into
account when decoding components into the pixmap. Combined
this resolves the issue that previously triggered ASAN.

Thanks to oss-fuzz for reporting.

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