Fixes unnatural black flickering in some scenes where encoding noise
pushes the background between 0 and 1 PQ. Adding a 1% PQ cutoff allows
censoring such values to 0.

I could also have hard-coded this constant but I decided it's simple
enough to expose, and it being tunable may help with tracking down
related issues in the future, and also allows users to opt out of this
functionality altogether.

Fixes: https://yabb.jriver.com/interact/index.php/topic,136378.msg950343.html#msg950343

Makes the options page a bit more readable.

This reverts commit e78c3512.

Very easy to slot into the new variable reference wrapper system.

Add const char examples, and also document the `ident` type for
completeness' sake.

We want to use the locale-invariant asprintf_c, matching the behavior of
the old GLSL() macro.

These are internally treated as integers, but have extra user-friendly
names associated with them. These names will be available in both rpn
exprs and as #defines inside the shader body, for convenience.

Closes: https://github.com/mpv-player/mpv/issues/12093

Including the new #pragma GLSL macro

For hooking the point immediately before alpha blending, for any shaders
that want to make a last-ditch image modification along the lines of
rotation, corner rounding, etc.

Instead of half-assing it with a #define, which prevents the use of
mipmapped textures (e.g. in externally imported samplers and custom
shaders).

Needed for #276

This marks the first time the entire syntax was documented outside of
the source code as reference. Hooray!

Code tidiness.

Disambiguate it slightly from rendering.md.

Using `mkdocs` + `material` theme because it was the nicest-looking
thing that I found with a few minutes of googling for alternatives.

Here's hoping I don't regret that decision.