Add foveation benchmark

[jorgeag-google]

The third of a series of CL that integrates the commits from:

https://github.com/bjoeris/bigwheels/tree/experimental-foveation

into BW main's branch.

The series will have 4 CL, this is the third one that integrates the following commit

Add foveation benchmark

Beside the foveation sample app, it also

• Adds ImmutableSamplers member to DescrptorBinding struct
• Refactor code from TakeScreenshot and adds a SaveImage member in Application class
• Adds a cmake function to add a GLSL shader target

[jorgeag-google]

I adress the simple changes, this is a save progress commit. I will continue tomorrow

[bjoeris]

lgtm

[apazylbe]

Haven't looked in detail at the project code, but otherwise lgtm.
Could you please update the PR description to actually describe the changes? When we refer to history it might not be as useful to know that this was one of the few changes to be merged. For example, describe changes to bigwheels itself (immutable samplers, application take screenshot) or the foveation benchmark itself.