[no squash] Extract image generation/cache from texturesource.cpp and some refactoring

[cx384]

Goal of the PR

• This is the followup PR of [no squash] Split up tile.cpp/tile.h and just a little refactoring  #14401
• The goal is to improve texturesource.cpp such that it only handles texture storage, and no source image caching or applying of texture modifiers.

How does the PR work?

• Everything related to image generation/modification and source images caching moved to imagesource.h/cpp
• Since those are the parts which can only be accessed from the main tread, it simplifies the tread safety code in texturesource.cpp, and imagesource.cpp doesn't need any mutex.
• The remaining texturesource.cpp gets refactored, but it's mostly modernizing and formatting, so there shouldn't be much behavioral difference.
• SourceImageCache does also get refactored a little bit, but the rest of imagesource.cpp stays the same, since it would be too much for this PR to also reactor it.
• Information to hopefully make it easier to review:
  • IImage is an image object, which can be modified.
  • "source image" is an IImage loaded from a texture file without any modification.
  • ITexture is made from an IImage for the driver to use. It doesn't get modified.

Does this relate to a goal in the roadmap?

2.2 Internal code refactoring

To do

This PR is Ready for Review.

(The next step is to refactor imagesource.cpp.)
(In the future, it may be good to replace const string reverences by string views everywhere, but this would involve changing many files.)

How to test

• See that the functions which moved to imagesource.cpp didn't change.
  (e.g. use git diff --color-moved <commit1> <commit2>)
• Read the refactored code.

[sfan5]

forgot to review the second commit

[cx384]

Thanks for the review, I resolved everything and rebased.

It is ready for review again.

[cx384]

Rebased to include the changes which #14448 made.