Add alternate ANIMDEF lump, RVFXANIM, for altering texture animation speed when reducevfx is on.

In order to address photosensitivity issues with certain animated textures, an alternate ANIMDEFs variant, RVFXANIM, has been created that is only loaded if reducevfx is on, and will replace the relevant animations in loaded ANIMDEFs. Any animation defined in a RVFXANIM lump is automatically clamped to a minimum animation speed value of 8 tics, in order to prevent malicious abuse. It otherwise functions identically to a normal ANIMDEF. Functions are added to automatically load or unload RVFXANIM lumps when reducevfx is toggled on or off. RVFXANIM should be whitelisted similarly to MUSICDEF.

A demonstration of RVFXANIM lumps for testing purposes is included below. This can also be considered a draft proposal for problem animations that should be addressed if this is merged. PhotosensHardcodeTest.pk3

Additionally, to demonstrate the effects of the change to P_GetMidtextureTopBottom, the below map is attached. ATestForTerrainMidtextureAnimation.pk3

For this test map, expected behavior on 2.3 is that the player will be blocked from taking the ramp over the midtexture while it is on its largest frame, but will be able to clear over it on the smaller frames. In this branch, the expected behavior is that the player will be blocked by the midtexture regardless of the frame and will appear to collide with "thin air." This is desired so that animated midtextures affected by RVFXANIM should have no risk of desyncing a game. (Animated textures with varying terrains are also included in this test map, but the behavior they were meant to test seems to already work as needed by this branch and required no changes - barring any surprises, they should always act as offroad despite several frames being defined otherwise.)

Additionally, as of this edit, the map includes an ACS script that calls for the texture names of each animated texture in the map on a constant loop each second. Expected behavior is that this will only report the first frame of the animation at all times for both textures (TASTOA01 and TASTOB01, respectively).

Should be ready for review now.