tests: add rgb[] alias in color_float

Individual struct fields are inconvenient to index into, yet most
operations on a color just repeat the same for each of RGB channel.
Being able to index into RGB avoids repeating the same code for each
channel.

Alpha channel is left as separate, since it is almost never handled the
same as RGB.

The union keeps the old .r, .g and .b addressing working. The static
asserts ensure the aliasing is correct.

For demonstration, two simple functions in color_util.c are converted.

Unfortunately initializers need to be corrected everywhere. Field .a is
not explicitly initialized because it is unused in these cases.

This change should make code easier to read.

This change requires gnu99 or c11 standard. gnu99 is already the default
in top-level meson.build.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>