Avoid saving to SRAM when loading preset or savestate

[idlechild]

Also fix a couple issues in the next-update branch.

Even though tile tables uses crossbank, the starting value of $E68800 was chosen so each tile table fits within a bank. Starting at $E68000 was causing some issues (try jumping to LN or bubble mountain preset). Also made freespace report the $800 bytes available at the start of bank $E6 (which I had forgotten existed, so nice to know about).

[InsaneFirebat]

If the goal is to cut down on SRAM writes overall, I would axe !sram_seed_x/y entirely. It was a weird idea to keep the seed rolling forever since it only cycles when the menu is open, but can be used without the menu ever being opened.

We should add a comment on the tilegraphics org. I figured it was just a remnant of things moving over time.

[idlechild]

Meh, it's fine to leave that.

This change came from a review I did of SRAM usage, where I was expecting to find we write to SRAM a lot but we actually weren't. The write-on-load was the only SRAM write I thought was dumb and could be improved.

We read from SRAM a lot but it's not a problem to do that.