The framebuffer size was reduced in f2c0cee, but this caused some niche
block layouts to write outside of the frame.
This could be fixed by adding checks to see if a block being predicted/
reconstructed is within the frame, but the branches introduced by that
reduce performance slightly. Therefore, it's better to keep the
framebuffer sized according to the decoded frame size in 8x8 blocks so
that any block can be decoded without bounds checking.
A test was added to ensure that this continues to work.
1422f7f904