Textscreen: support for forcing software rendering

[fsufitch]

Added a TXT_SetForceSoftwareRenderer function to set a global configuration flag for textscreen, plus support for a TXT_FORCE_SOFTWARE_RENDERER environment variable. Setting either of these has the same effect on how SDL_CreateRenderer is called as the force_software_renderer option in Choco's config does.

Additionally, Setup and Endoom call TXT_Init, call TXT_SetForceSoftwareRenderer as appropriate given the configurations they are operating under.

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Motivation for this change: I am using Chocolate Doom on a Raspberry Pi 1A, and while SDL can initialize its regular OpenGL environment, it is excruciatingly slow (0.5-1 FPS) even for something as simple as Textscreen. Forcing it to use software rendering instead of pretending to be a GPU fixes this.

Thus, this change greatly improves support of very low spec hardware.

[turol]

This doesn't match coding style

[turol]

Looks OK other than the code style. Do you want to rebase/squash this into one commit or should it be done when merging?

[fragglet]

Is there any reason not to just always use a software renderer for textscreen? It doesn't have the same performance requirements that the main binary has after all. Might be a simpler solution.

[turol]

There was a PR that made the textscreen and main game share the same renderer. How will that be affected by this change or always forcing software renderer for textscreen?

[fsufitch]

Is there any reason not to just always use a software renderer for textscreen? It doesn't have the same performance requirements that the main binary has after all. Might be a simpler solution.

This makes a lot of sense. I cannot envision any circumstance where textscreen needs hardware accel.

Looks OK other than the code style.

Sorry, I am very much a Go / Python / Typescript dev, not C. I will fix whatever is going on, but I'm not sure what I got wrong. Hints?

[turol]

Check HACKING.md for coding style documentation. If you rebase this on latest master there should now be a linting CI check that should comment on any style violations.