No longer pre-generating the download link target

Generating this at paint time is a waste of time, since the
download link might never be clicked. Instead, the data URL
is generated when the download link is clicked.
Cleaning up controls a bit
Adding labels for slider values
Removing black-and-white-constant-line-count option

Pointless because it's just a black blob. Also changing
the color vs. black and white and constant vs. proportional
radio buttons to a drop-down, which is simpler and looks better.
Improving image downloading
Minor header visual improvements
Using same file input for intro and normal screens

Avoids code duplication, and also causes correct file name to
be shown in the input after the first upload.
Making header just a bit more responsive
Making file uploads compatible with FireFox
Adding more specific performance logging