Fix desired size space allocation not respecting minimum sizes correctly

[StarryWorm]

What problem(s) does this PR solve?

• Follow up to Fix issues with desired size system #120640

In that PR, I made BoxContainer and GridContainer (and FlowContainer, but that one isn't bugged) allocate space to children whose desired sizes were greater than their minimum sizes.
As described in the PR, the flow would go:

1. Give each child its minimum size
2. From the remaining space, distribute to children with a desired size
3. If any, from the remaining space, distribute to children with the SIZE_EXPAND flag

The problem is, in both cases, the "remaining space" computed for step 3, which I used for step 2, also includes the space occupied by the minimum size of the children who have the SIZE_EXPAND flag. This is done such that their final sizes are computed irrespective of their minimum sizes, and only take into account the stretch ratios.
However, using that in step 2 resulted in the children in step 2 overallocating, using the minimum size of the expanding children as free space to stretch into. This is completely erroneous and results in the step 2 children taking up more space than is available.

The images below use HBoxContainer, the project has both, but they look the same, so I only put images for one.
Before:

After PR:

Project:
respect-desired-sizes-mrp.zip

Additional information

The project additionally contains the previous tests from #120640 to ensure no regressions against that PR.

This bug slipped through because of a lack of test coverage for the new system, which I should have added earlier. I will work to expand test coverage for these containers so that this kind of oversight does not happen again.

[YeldhamDev]

Fixes the issue. And indeed, GUI tests for sizing would help a lot in minimizing those regressions.