reshade: Summarise code-section of `ReshadeUniform::{PingPongUniform,RandomUniform}()`

[tcoyvwac]

Hi there! :smile:

Following the positive response of PR:#998, and a followup to PR:#1260, this PR condenses and summaries some common code-fragments in ReshadeUniform::{PingPongUniform,RandomUniform}'s constructors - ().

As usual, I kept the commit history for code-review purposes and ease however please feel free when this PR is complete, to condense / squash everything into one commit for git-history reasons if you like!

Please note:
There are additional points and questions discovered and raised in this PR. They are listed in a further comment ahead here.

Of course, happy to answer any questions and do my best explaining the design of the changeset.

Thanks and hope this helps! :smile: