Fixes to iOS render loop and HUD overlay.

[billhollings]

• Refactor Platform::main_loop_frame() and main_loop(),
  to reduce fragility, and avoid unnecessary code duplication.

• Avoid initial app_requested() test in main_loop_frame(),
  as it was causing looping to stop after one frame.

• Since main_loop() is not a virtual function, remove
  declaration comment indicating it can be overridden.

• Set iOS view to have a black background, to ensure
  transparency in HUD overlay displays in full on device.

• Fix rendering scaling in IosWindow::get_dpi_factor().
  Previous value was causing HUD overlay to render magnitudes too large.

• Add macOS files to .gitignore.

I created this from a branch on this repo instead of a separate fork, because I was not expecting to need to make a fix.

Fixes #1121
Fixes #1131

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[asuessenbach]

You should at least resolve the failing Quality Check.

[billhollings]

You should at least resolve the failing Quality Check.

If tried many times.

1. For some reason, clang-format works slightly differently on macOS than on other platforms, so running ./scripts/clang_format.py main ends up changing more than expected, and breaks the CI test here.
2. The CI error messages are incomprehensible to me, so I'm not able to decipher what it's complaining about, in order to simply fix it manually.

[gpx1000]

https://github.com/KhronosGroup/Vulkan-Samples/actions/runs/10461719961?pr=1120

If you scroll to the bottom you can grab the diff that can be directly run and hopefully fix the issue.

[asuessenbach]

For some reason, clang-format works slightly differently on macOS than on other platforms

Indeed, it should remove white spaces at the end of a line, and it should replace spaces with tabs.
This patch shows the difference I would get when running clang-format: 0001-formatting.patch

[billhollings]

@gpx1000 @asuessenbach

Thanks for the help on code formatting. I've got this formatting correctly now.

Plus I've added a couple of other iOS fixes, and updated the title and description above.

[asuessenbach]

By code inspecting and running on Windows: LGTM.

[SRSaunders]

LGTM. Did full batch runs for iOS + macOS and both main_loop_frame() and main_loop() worked correctly. Still need fixes for non-success return codes in iOS viewDidLoad() and renderLoop(), but these can be dealt with in a future PR.

[billhollings]

LGTM. Did full batch runs for iOS + macOS and both main_loop_frame() and main_loop() worked correctly. Still need fixes for non-success return codes in iOS viewDidLoad() and renderLoop(), but these can be dealt with in a future PR.

@SRSaunders Would you mind giving this a formal approval, so it can be merged, please?

Once the approvals are in, what is the process for merging? Do I do that? Or is there a designated person responsible for merging?

[gpx1000]

We usually ask @marty-johnson59 to handle the merging. We usually merge on 3; but Marty, I think this is safe to merge on 2.

[SRSaunders]

@SRSaunders Would you mind giving this a formal approval, so it can be merged, please?

@billhollings I don't have approval rights for this project. I am happy to provide feedback, but I cannot mark the PR as ready for merge. As you see above, @gpx1000 has that ability.

[marty-johnson59]

Thanks. Given this is Mac (we have limited folks who can test), I'm happy to merge w/ 2. We can revert if needed.