Improve frametiming for linux capture

[gschintgen]

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Description

This commit computes the time of the next screen capture based on the current frame's theoretical time point instead of the actual capture time. This should be slightly more precise and lead to better frame timing.

I made the same change in three parts of the codebase:

• twice in kmsgrab.cpp (display_ram_t and display_vram_t). IIUC these correspond to capture to system RAM for software encoding and capture to VRAM for hardware encoding respectively. Please correct me if I'm wrong; I did not try to understand the whole pipeline.
• once in x11grab.cpp which presented the same issue.

At this point I did only preliminary testing of the kmsgrab & VA-API case on an AMD 6650. The results were very encouraging with a precise 60.0 Hz instead of the previous average of 59.94 Hz. (See #2286 for details.) I'll vary my testing over the next few days and report back. Any feedback and review is welcome, of course.

If I'm not mistaken this fix should benefit all Linux users with the exception of NvFBC configurations.

Screenshot

Issues Fixed or Closed

Fixes #2286

Type of Change

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Dependency update (updates to dependencies)
• [ ] Documentation update (changes to documentation)
• [ ] Repository update (changes to repository files, e.g. .github/...)

Checklist

• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [ ] I have added or updated the in code docstring/documentation-blocks for new or existing methods/components

Branch Updates

LizardByte requires that branches be up-to-date before merging. This means that after any PR is merged, this branch must be updated before it can be merged. You must also Allow edits from maintainers.

• [x] I want maintainers to keep my branch updated