This works around the problem that crates pushed to crates.io cannot
reference files outside of their crate directory.

Fixes #1864.

This adds the option to specify the color of the visual bell using the
`visual_bell.color` configuration setting.

This is done by rendering a big quad over the entire screen, which also
opens up options to draw other arbitrary rectangles on the screen in the
future.

Since landing the patch adding transparency support to Alacritty,
there's been an issue where othewise solid background cells were also
being rendered partially transparent. Now, all filled background cells
are rendered fully opaque.

Some logic was added to support discarding filled backgrounds which had
the same color as the default background. This means that, if the
default background is #000 and a cell has that background, it will never
be rendered opaque. This may not be correct.

Note that many truecolor vim color schemes print spaces for default
colored background cells. Performance can be dramatically improved by
using ctermbg=NONE guibg=NONE to skip rendering those cells.

The option is an Alpha struct that ensures that the contained float is
between 0.0 and 1.0. Background colors are multiplied by the opacity
to properly alpha blend them.

We previously had a hard-coded value for aligning glyphs within cells.
The font descent is now used, and the offset should be correct by
default.

This commit adds support for a visual bell. Although the Handler in src/ansi.rs
warns "Hopefully this is never implemented", I wanted to give it a try. A new
config option is added, `visual_bell`, which sets the `duration` and `animation`
function of the visual bell. The default `duration` is 150 ms, and the default
`animation` is `EaseOutExpo`. To disable the visual bell, set its duration to 0.

The visual bell is modeled by VisualBell in src/term/mod.rs. It has a method to
ring the bell, `ring`, and another method, `intensity`. Both return the
"intensity" of the bell, which ramps down from 1.0 to 0.0 at a rate set by
`duration` and `animation`.

Whether or not the Processor waits for events is now configurable in order to
allow for smooth drawing of the visual bell.