color: Fix formatting.

Clearly Python-inspired and in one case nearly a bug >:(
color: Implement al_color_html_to_rgb and al_color_html more strictly.

Make al_color_html_to_rgb return a success code, and document the format
explicitly.  Make al_color_html return transparent black if the string
could not be parsed.

This patch should NOT be backported to the 5.0 branch.
docs: Pedanticly describe result buffer for al_color_rgb_to_html.