Improve metadata retrieval performance

On older or single-core PCs, metadata parsing represents a significant
chunk of the image load process.  This latest commit introduces a number
of changes geared toward improving that performance.

Profiling showed that PD was using too small of a buffer for receiving
ExifTool output, and that it was waiting too long between stdout polls.
These have both been adjusted to their largest/smallest (respective)
practical limits, and as a result images with large amounts of metadata
(particularly RAW images or CMYK images with ICC profiles) should now
load much more quickly.

I've also adjusted the way PD load process pauses while waiting for
metadata parsing input to finish.  Instead of a blind DoEvents loop, we
now use a more intelligent mix of Sleep and DoEvents to allow ExifTool
greater access to the scheduler on older PCs.  I also check for program
termination in the wait loop now, so if the user decides to close
PhotoDemon while we're waiting for ExifTool input, we no longer leave a
running loop in the background, waiting for input that will never
arrive.

The user interface has also been forcibly locked down during metadata
parsing to avoid re-entrance issues.