Texture updates and fixes

[GeorgePieVG]

• Flat512: combined with floor30 to give a less faded look.
• Flat520-523: made smoother and brighter.
• Fltflww: stretched to 65 pixels tall like in the original Heretic to fix the tutti frutti.
• Rhtp flats: added color variety.
• Rhtp30, 42: uses blasphemer marble baseboard.
• Snake1: added eyes.
• Wall25, 502, 503, 504, 505, 516, 517 are now the same size as Heretic patches, their texture definitions are now the same as in Heretic. Wall25 is based on the patch by Korp from Freedoom attic. Cross is now a standalone patch.
• All the marble patches and flats got smoother.
• Added texture definition for wlgore01.