Fix render issues, re-enable buffer compaction

[kazcw]

Pull Request Description

• Fix handling of a Symbol with no instances. We had logic to call context.draw_arrays (non-instanced) if instance_scope().size() == 0. This was incorrect: All Symbols are instanced symbols, and should only be drawn if at least one instance exists. This didn't cause a problem before because it was unreachable: We never instantiated a Symbol without creating at least one instance, and without buffer compaction it was not possible for instance count to decrease. Fixed by skipping all draw-related GL calls if there are no instances (this might also improve performance in some conditions).
• Fix an incorrect OptVec iteration pattern (would cause a crash in conditions not currently reachable).
• Re-enable buffer compaction.

Important Notes

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
  Scala,
  Java,
  and
  Rust
  style guides. In case you are using a language not listed above, follow the Rust style guide.
• All code has been tested:
  • Unit tests have been written where possible.
  • If GUI codebase was changed, the GUI was tested when built using ./run ide build.