In GimpProjection, use an adaptive chunk size when rendering the
projection asynchronously, rather than using a fixed chunk size.
The chunk size is determined according to the number of pixels
processed during the last frame, and the time it took to process
them, aiming for some target frame-rate (currently, 15 FPS).  In
other words, the chunks become bigger when processing is fast, and
smaller when processing is slow.  We're currently aiming for
generally-square chunks, whose sides are powers of 2, within a
predefined range.

Note that the chunk size represents a trade off between throughput
and responsiveness: bigger chunks result in better throughput,
since each individual chunk incurs an overhead, in particular when
rendering area filters or multithreaded ops, while smaller chunks
result in better responsiveness, since the time each chunk
individual takes to render is smaller, allowing us to more
accurately meet the target frame rate.  With this commit, we aim to
find a good compromise dynamically, rather than statically.

The use of adaptive chunk sizes can be disabled by defining the
environment variable GIMP_NO_ADAPTIVE_CHUNK_SIZE, in which case we
use a fixed chunk size, as before.