Speed Optimizations

Have replaced the use of many Int32Arrays with a single 'matrix'
Int32Array for each component. Created new versions of functions that
interact with that array.

Removed the 'buffer copy' code at the end by writing
getData_rgb_to_rgba_buffer, which writes directly to an RGBA buffer.

In the test I did, observed decoding time decreased from 3300 to 760ms.

There are further optimizations that can yet be made, to do with better
use of typed arrays, and not allocating various variables multiple
times, but reusing space in typed arrays.