Fix SDL color encoding.

Little-endian + RGBA means that red is located at the lowest-order bits,
not the highest.

The hello_world_sdl test is now portable, because we no longer write to
individual bytes.

We also add a test for a completely blank screen, which should be black, not
red.

Closes #1287, #761, #765.
Make browser tests nicer to run.

A bunch of misc changes, including fixing compile-time warnings, and
muting the Python server request log.