Merge new upstream changes

[caseyjoy]

Cherry pick some new merged upstream Milkytracker changes:

• [x] 3c60de3b4376df1d105a9efaf581b9adf8d3ab36 Fix About screen crash caused by missing null terminator
• [x] 59f01750f1492fe4f558f73813b056303a9fafe0 Add quarter sine generator
• [x] 66c31bf66dad2c76f445f0f0ad814a1d4702f509 Merge pull request https://github.com/milkytracker/MilkyTracker/pull/273 from vanjac/fix-lowres
• [ ] Possibly 50267642dffe12b6c4c2aa2144c0f110baea0d02 Remove executable bit on source files (check for if the executable bit is still set first, this might also be a windows executable bit, so look for that too)
• [x] 6d5dafa489bcb576e0a2a3c5f2b30c32b033d64d fix segfault when passing invalid filename on command line (check to see what's going on here first, I would have expected the loader to check to see if it can load a file or not)

[caseyjoy]

Merged commits, skipped the one that removed executable bits from files for now.