Allow 64DD disks to boot without N64 cart

this PR is still WIP,

the main goal is to allow 64DD games to boot without requiring a N64 cart ROM.

I opened this PR early because I wanted some feedback on the changes I've made so far (mainly in open_rom & main_run)

the main things TODO are:

• rework is_dd_rom
• split load_dd_disk into separate functions,

note that this already functions as-is on ndd roms

cc @bsmiles32

Not a dev here, but I'm asking you anyways this.

Is this PR compatible with https://github.com/mupen64plus/mupen64plus-core/pull/762? Did you have some coordination with @LuigiBlood on this? AFAIK he's currently working on 64DD too.

Not a dev here, but I'm asking you anyways this.

Is this PR compatible with #762? Did you have some coordination with @LuigiBlood on this? AFAIK he's currently working on 64DD too.

it's not compatible right now, I think I'll wait for #762 to be finished & merged before I continue work on this PR, LuigiBlood is aware of this PR but I'm not entirely sure if he wants to implement the functionality this PR wants to implement, if he wants to, then he could take inspiration from this PR but that's upto him.

As long as you both are aware of each other's PR, that's good. Thanks for your work on this, I think this is a nice feature to have on M64+.

Feel free to work on this PR after the other will be merged probably soon as this functionality would be added later as @bsmiles32 would prefer splitting features in different PRs as he said in #762

This subject is a little bit more tricky unfortunately because of how the core works. I'll try to get back to you (either with a PR or with more precise comments) after the current DD rework.

edit: As a workaround current limitation, I select a Japanese cart ROM which has compatible ROM settings in the DB. For instance, to test Mario Studio with Transfer Pak I use the Perfect Dark (J) ROM with Mario Studio Disk.

ping @bsmiles32, any ideas or progress? I could work on it myself if you give me some pointers on how to do it 'properly'

I'm still interested to see updates about this PR.

Superseded by https://github.com/mupen64plus/mupen64plus-core/pull/983