JsCharacterGeneratorScript: full-size draws

Passing -1 for w or h allows drawing the whole width or height of the asset,
instead of just a portion.
RawFrame: fix leaked fd when loading image files
PngSubprocessCharacterGenerator: add error message

Print an error message if the PNG image fails to decode to a RawFrame
for some reason.
js_keyer_server.rb: return status JSON

previously, nothing was returned, causing confusion to API users
svg_http_keyer: roll in static_svg.rb functions

Add command line options to svg_http_keyer.rb to allow an initial key
to be loaded and brought up. This allows static_svg.rb to be eliminated.