Fix CUDA RGBA to NV12 conversion

[w0utert]

Description

On linux hosts with Nvidia GPU and CUDA support enabled, a CUDA kernel is used to convert captured RGBA frames to NV12 before encoding. This kernel contained a bug affecting image quality, in particular when rendering high-contrast colored text and sharp lines. See [1] for more information.

This commit fixes the format conversion kernel by taking 2x2 RGBA blocks to generate 4 luma (Y) values and 1 chroma (UV) pair, ie: 12 bits per pixel YUV420 (NV12). Previous code incorrectly generated 1 UV pair for every 2 pixels.

[1] https://github.com/LizardByte/Sunshine/issues/154

Screenshot

Issues Fixed or Closed

• Fixes #154

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)

Checklist

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

[ReenigneArcher]

Does this completely fix 154? Or partially?

[w0utert]

Yes, it completely fixes #154