Added new scaling modes to splash screen

[samuelpedrajas]

The only way to scale the boot splash image is the option "Application/Boot Splash/fullsize" in the project settings. This PR adds 4 new ways of scaling: keep_width, keep_height, cover and expand and keeps the previous one (calling it full_size).

The old fullsize option looked like this:

And I replaced it for a new option I called "scale":

Here I show some pictures to illustrate how each scaling mode works. In each case an icon with size 64x64 is used:

Scaling Mode	Horizontal (960x540)	Vertical (540x960)
disabled
keep_width
keep_height
cover
expand
full_size

The last one scales in the same way as the previous "fullsize" boolean option.

I updated both rasterizer_gles2.cpp and rasterizer_gles3.cpp. I also realised that the old "fullsize" option was not working when using GLES2.

This is my first PR so any tips, suggestions, recommendations... about anything are very welcome, specially if it's code-related.

[reduz]

Problem with this PR is that it breaks compatibility, so it can't be merged until 4.0.. maybe adding another property like expand_mode ?

[samuelpedrajas]

Sorry for the late response.

I could keep the "fullsize" property and create a new dropdown called "expand_mode" which would only take effect when the "fullsize" property is true. The dropdown could have all the options listed in the description of this PR, renaming the choice "full_size" to just "keep". Would that be fine and would that keep compatibility?

Also, I removed my fork and now the PR is shown as commited from an "unknown repository". I've been researching and it looks like I will have to make a new PR. Sorry for this! My lack of experience on GitHub... ^^'.

[samuelpedrajas]

I finally opened a new PR (because as I said I can't edit this PR anymore) here: #22488. I think that now this one becomes obsolete and can be rejected.

[akien-mga]

Closing as superseded by #22488.