Fix layout gap

[joshka]

Fixes #367

Note: this builds on #405, so it has the commits for that here. Look at just the last commit in this PR to understand it. The relevant change is just the addition of round():

ratatui/src/layout.rs

Lines 453 to 454 in 2364d9b

	let start = changes.get(&element.start).unwrap_or(&0.0).round();
	let end = changes.get(&element.end).unwrap_or(&0.0).round();

Note each of the test changes is a subjectively better rendering choice. I'm not 100% sure if there are some tests that would make it easier to see this change's effect rather than those that are already changed.

I updated the colors example to use a ratio instead of a percentage as this now renders correctly without gaps.

Before (using constraint: vec![Ratio(1,8); 8] )

After:

[codecov]

Codecov Report

Merging #408 (a1fa41d) into main (f4ed3b7) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #408      +/-   ##
==========================================
- Coverage   86.14%   86.13%   -0.02%     
==========================================
  Files          40       40              
  Lines        9256     9240      -16     
==========================================
- Hits         7974     7959      -15     
+ Misses       1282     1281       -1     

Files Changed	Coverage Δ
src/layout.rs	96.28% <100.00%> (+0.05%)	⬆️

[joshka]

Rebased on main to pickup the bug fix in #408

Removed the no longer necessary comment in tests.

I also removed the code that checked the last element filled the area as the required constraint and corrected rounding take care of that fully now. I can't think of any possible way to trigger this code to mis-allocate the last constraint to less than the full amount that wouldn't also panic earlier due. Prior to this change, this was possible when the code took the floor of the start and floor of the width. E.g.: with area of 10:

• when the last element is (start: 9.4, width: 0.6), the old start and width were (start: 9, width: 0). With this change that becomes (start: 9, width: 1).
• Similarly for (9.6, 0.4) old was (9, 0) and this change becomes (10, 0), with the extra space allocated to the previous element.