NEEDS REVIEW

continuation of #5037
resolves #4729 

renders all shaders to the default buffer and then copies it to the
designated custom shader texture.

this is a draft pr because:
- it introduces a new shader "pipeline" which doesnt fit in with how the
system was designed to work (which is only rendering to the fbo)
- im not sure if this is the best way to achieve shaders being able to
sample their output while also drawing to the screen. the cusom fbo
(previous implementation) was useful in that it modularized the custom
shader stage in rendering

---------

Co-authored-by: Mitchell Hashimoto <m@mitchellh.com>

It seems like the raw data version of the kitty graphics transmit
operation is meant to be unassociated (aka straight) alpha, though I
can't find any definitive documentation either way- but in any case
unassociated alpha is more common in image formats and makes the
handling easier for the rest of it.

Also removed a redundant call to `decode_frame_config`, since it's
called implicitly when we call `decode_frame` right after.

This commit is quite large because it's fairly interconnected and can't
be split up in a logical way. The main part of this commit is that alpha
blending is now always done in the Display P3 color space, and depending
on the configured `window-colorspace` colors will be converted from sRGB
or assumed to already be Display P3 colors. In addition, a config option
`text-blending` has been added which allows the user to configure linear
blending (AKA "gamma correction"). Linear alpha blending also applies to
images and makes custom shaders receive linear colors rather than sRGB.

In addition, an experimental option has been added which corrects linear
blending's tendency to make dark text look too thin and bright text look
too thick. Essentially it's a correction curve on the alpha channel that
depends on the luminance of the glyph being drawn.

The cimgui version listed does not match the upstream commit or the
vendored cimgui files
Checking the upstream `git log` the commit corresponds to `commit
e391fe2e66eb1c96b1624ae8444dc64c23146ef4 (tag: v1.90.6-docking)` however
the `build.zig.zon` is outdated.
The vendored cimgui files also contain the header
```cpp
// This file is automatically generated by generator.lua from
// https://github.com/cimgui/cimgui based on imgui.h file version "1.90.6" 19060
// from Dear ImGui https://github.com/ocornut/imgui with imgui_internal.h api
// docking branch
```

I wasn't too clear with what the comment meant:
```
// This should be kept in sync with the submodule in the cimgui source
// code to be safe that they're compatible.
```
and assumed it was referring to the vendored cimgui files, added a
comment pointing out where to find the cimgui source mentioned.