Add screenshot capture to benchmark system for automated shader testing

[LegNeato]

• Add saveScreenshot() method to capture swapchain images as PPM files
• Extend benchmark class to support screenshot capture at specified frame
• Add command-line flags --screenshot and --screenshotframe

This enables automated differential testing (not included in this PR, but trivial to add on top. I am doing so in our Rust fork)

[LegNeato]

(note I am not a C++ programmer so feel free to give pointers and corrections)

[LegNeato]

FWIW, here is output from my script that scripts the differential testing. Let me know if you want me to put up the script in a future PR:

Testing example: triangle
  Running with glsl shaders...
  ✓ glsl completed
  Running with hlsl shaders...
  ✓ hlsl completed
  Running with slang shaders...
  ✓ slang completed
  Running with rust shaders...
  ✓ rust completed
  Comparing outputs...
  ✓ glsl and hlsl are identical
  ✓ glsl and slang are identical
  ✓ glsl and rust are identical
  ✓ hlsl and slang are identical
  ✓ hlsl and rust are identical
  ✓ slang and rust are identical

Testing example: texture
  Running with glsl shaders...
  ✓ glsl completed
  Running with hlsl shaders...
  ✓ hlsl completed
  Running with slang shaders...
  ✓ slang completed
  Running with rust shaders...
  ✓ rust completed
  Comparing outputs...
  ⚠ glsl and hlsl differ
  ⚠ glsl and slang differ
  ⚠ glsl and rust differ
  ✓ hlsl and slang are identical
  ⚠ hlsl and rust differ
  ⚠ slang and rust differ

Testing example: computeshader
  Running with glsl shaders...
  ✓ glsl completed
  Running with hlsl shaders...
  ✓ hlsl completed
  Running with slang shaders...
  ✓ slang completed
  Running with rust shaders...
  ✓ rust completed
  Comparing outputs...
  ✓ glsl and hlsl are identical
  ✓ glsl and slang are identical
  ⚠ glsl and rust differ
  ✓ hlsl and slang are identical
  ⚠ hlsl and rust differ
  ⚠ slang and rust differ

[SaschaWillems]

Will first need to test if this actually works. I did something similar a few years ago, but ran into issues with sync on some platforms/devices.

No need for the differential testing script from my side though. I like to keep CI small and fast.

[LegNeato]

Sounds good, no rush! Looks the differential testing isn't that useful on a lot of the output anyway due to float differences. I only tested this on mac with MoltenVK FWIW (which means some shaders were not tested of course).