Use AUDIO_VIDEO_ENABLE flags to skip rendering when not needed by the frontend

[msheehan79]

This PR adds some checks based on the RETRO_ENVIRONMENT_GET_AUDIO_VIDEO_ENABLE macro to skip the rendering portion of video or audio when not required, in order to reduce the overhead when running features such as Runahead.

I did some benchmark testing on this PR using the method detailed below. In addition, I also just used the core as I normally would to play and did not notice any regressions during my playback.

Benchmarking:

I used Hyperfine to run the core in headless mode, using the following .cfg changes from default:

audio_driver = "null"
config_save_minimal = "true"
log_to_file = "true"
log_verbosity = "true"
rgui_show_start_screen = "false"
run_ahead_enabled = "true"
run_ahead_frames = "4"
run_ahead_secondary_instance = "false"
video_driver = "null"

Test Method: Used an average of 10 runs of the original vs pr core, calculated for 5000 frames, with 1 warmup run. I used 4 frames of runahead for the bulk of my testing.
Example command used: hyperfine -w 1 --runs 10 -n "Current Core" "retroarch.exe -L mame_libretro_current.dll C:\emulation\roms\arcade\mame\4dwarrio.7z -v --config benchmark.cfg --max-frames 5000" ^ -n "Updated Core" "retroarch.exe -L mame_libretro_updated.dll C:\emulation\roms\arcade\mame\4dwarrio.7z -v --config benchmark.cfg --max-frames 5000"

Results
The actual FPS gain varied quite a bit by game, but at no point did I see a regression or drop in FPS with this enabled.

Game	Time (Sec) (Original Build)	Time (Sec) (PR Build)	FPS (Original Build)	FPS (PR Build)	% Diff
pacman	7.156	4.902	698.7144	1019.992	45.98%
strider2	60.613	59.382	82.49055	84.2006	2.07%
005	9.404	7.126	531.6886	701.6559	31.97%
gdarius2	145.945	138.373	34.25948	36.13422	5.47%
sf2ce	22.428	19.448	222.9356	257.0958	15.32%
aquajet	121.092	103.938	41.29092	48.1056	16.50%
batsugun	38.214	35.622	130.8421	140.3627	7.28%
berzerk	20.942	14.965	238.7547	334.1129	39.94%
vf2	109.918	102.166	45.48846	48.93996	7.59%
4dwarrio	30.828	25.599	162.1902	195.3201	20.43%

AI Disclosure I used Gemini Pro 3.1 to help me get a better understanding of the libretro/MAME runloop and to suggest some possible locations where we could skip rendering, it was used a guideline only. All code was committed by me and all tests run myself, not using AI agents,