Wayland screencast: refactor BuildFormat into separate functions

Split the monolithic BuildFormat() function into three specialized
functions for better flexibility and code reuse:

- BuildBaseFormatParams(): Builds core format parameters for a
  single video format
- BuildBaseFormat(): Builds format parameters for all supported
  pixel formats without DMA-BUF modifiers
- BuildFullFormat(): Builds format parameters including DMA-BUF
  modifiers with fallback formats

Bug: webrtc:461543454
Change-Id: I2ce18f0cb869c2ab111bc66a4e71d99c9da977d4
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/433560
Reviewed-by: Tomas Gunnarsson <tommi@webrtc.org>
Commit-Queue: Jan Grulich <grulja@gmail.com>
Reviewed-by: Alexander Cooper <alcooper@chromium.org>
Cr-Commit-Position: refs/heads/main@{#46727}