Don't allocate when decoding uncompressed and block-based formats #9
Description
The block-based decoders (BC + sub-sampled) and all rectangle decoders currently process the image line by line. This requires allocating on the heap to store a line. Of course, only one allocation is used for all lines, but it would still be better to have none.
It's possible to change all of those decoders to decode images with just a bit of stack memory. Instead of reading an entire line, split each line into chunks small enough to fit into a fixed-size buffer on the stack.