optimize_png: Improve missing tool output

Print missing tool instead of showing a generic message.

Signed-off-by: Luis Cruz <ljc2491@gmail.com>
optimize_png: Attempt to make install dir for PNGOUT

Include this for completeness, in case optipng is installed anywhere
else than optipng's 'make install'.

Signed-off-by: Luis Cruz <ljc2491@gmail.com>
optimize_png: Add help option

Invoke help with:
    optimize_png -h

Signed-off-by: Luis Cruz <ljc2491@gmail.com>
optimize_png: Improvements & fix-ups v2

Added support for using pngquant for lossy compression.
    !! Lossy compression will only be used if the resulting PNG is
       smaller !!
Changed PNGOUT version to a variable (updates are questionable, but
if they do come, we'll be ready).
Improve on options (clean up tailing space when only using -a option).
Robustly show which tools are missing.
Show changes in files size after completing.
Add some exits where appropriate for completeness.

NOTE: While using a lossy compressor is not efficient for ALL PNGs,
the overall gains in reduction cannot be ignored.

Test of ~650 PNGs (from an Android theme)

Lossy > Lossless:

    Results:
        Old size: 1803336 bytes
        New size: 1082205 bytes
        Reduced:  721131 bytes (60% of original total size)

Only Lossless:

    Results:
        Old size: 1803336 bytes
        New size: 1520335 bytes
        Reduced:  283001 bytes (84% of original total size)

Signed-off-by: Luis Cruz <ljc2491@gmail.com>