I also put in a missing `defined (__APPLE__)` in d_netcmd.h related to cv_mouse2opt

Also removed a redundant `!defined (__APPLE__)` in d_main.c

Remove all remaining traces of the following macros for the obsolete DOS port, which were missed previously:
* `DJGPP`
* `__DJGPP__`
* `DJGPPDOS`
* `PC_DOS`
* `WATTCP`

May get rid of `MSDOS` later once I get word on whether I should kill it or not

- Added render flags (see r_defs.h)
- Implemented floor splats
- Drop shadow sprite rendering through render flags

(cherry picked from commit 3437b0690a3f4278e3ecc657102a126a3e2f3d13)

(cherry picked from commit 7a05a8acf73ac0f012135339447c9a935e3b7939)

WIP because I don't know if this wording is right for the error.

CMake support. Srb2win.rc still needs touchy, whatever.

Introducing Marathon Run. (I was going to call it Marathon Mode, but NiGHTS Mode being right next to it on the menu looked terrible.)

Basically a dedicated Record Attack-like experience for speedrunning the game as a continuous chunk rather than ILs. Has several quality of life features.

Benefits include:
* An unambiguous real-time bar across the bottom of the screen, always displaying the current time, ticking up until you reach the ending.
* Disable the console (pausing is still allowed, but the timer will still increment).
* Automatically skip intermissions as if you're holding down the spin button.
* Show centiseconds on HUD automatically, like record attack.
* "Live Event Backups" - a category of run fit for major events like GDQ, where recovery from crashes or chokes makes for better entertainment. Essentially a modified SP savefile, down to using the same basic functions, but has its own filename and tweaked internal layout.
* "spmarathon_start" MainCfg block parameter and "marathonnext" mapheader parameter, allowing for a customised flow (makes this fit for purpose for an eventual SUGOI port).
* Disabling inter-level custom cutscenes by default with a menu option to toggle this (won't show up if the mod doesn't *have* any custom cutscenes), although either way ending cutscenes (vanilla or custom) remain intact since is time is called before them.
* Won't show up if you have a mod that consists of only one level (determined by spmarathon_start's nextlevel; this won't trip if you manually set its marathonnext).
* Unconditional gratitude on the evaluation screen, instead of a negging "Try again..." if you didn't get all the emeralds (which you may not have been aiming for).
* Gorgeous new menu (no new assets required, unless you wanna give it a header later).

Changes which were required for the above but affect other areas of the game include:
* "useBlackRock" MainCFG block parameter, which can be used to disable the presence of the Black Rock or Egg Rock in both the Evaluation screen and the Marathon Run menu (for total conversions with different stories).
* Disabling Continues in NiGHTS mode, to match the most common singleplayer experience post 2.2.4's release (is reverted if useContinues is set to true).
* Hiding the exitmove "powerup" outside of multiplayer. (Okay, this isn't really related, I just saw this bug in action a lot while doing test runs and got annoyed enough to fix it here.)
* The ability to use V_DrawPromptBack (in hardcode only at the moment, but) to draw in terms of pixels rather than rows of text, by providing negative instead of positive inputs).
* A refactoring of redundant game saves smattered across the ending, credits, and evaluation - in addition to saving the game slightly earlier.
* Minor m_menu.c touchups and refactorings here and there.

Built using feedback from the official server's #speedruns channel, among other places.