UpdateImGuiTexture now handles creating font textures and modifying them.

Co-authored-by: TellowKrinkle <tellowkrinkle@gmail.com>

Video: add R10B10G10A2 and R16G16B16A16F texture types

Enable through command line options:
-C Graphics.Settings.TexturePNGCompressionLevel=[0-9]

Or from GFX.ini:
[Settings]
TexturePNGCompressionLevel=[0-9]

@see #10792

Almost all the virtual functions in Renderer are part of dolphin's
"graphics api abstraction layer", which has slowly formed over the
last decade or two.

Most of the work was done previously with the introduction of the
various "AbstractX" classes, associated with texture cache cleanups
and implementation of newer graphics APIs (Direct3D 12, Vulkan, Metal).
We are simply taking the last step and yeeting these functions out
of Renderer.

This "AbstractGfx" class is now completely agnostic of any details
from the flipper/hollywood GPU we are emulating, though somewhat
specialized.

(Will not build, this commit only contains changes outside VideoBackends)

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.