Fix DynamicTextureAtlasBuilder incorrectly padding added textures.

[andriyDev]

Objective

• Assets as entities v0 #22939 has been introducing diffs in the testbed_ui Text example, but the only thing that was changing was the order that glyphs are allocated to the font atlas. The font atlas should be resilient to this and produce no diffs in that case.
• Another PR had similar diffs and also seemed to be due to changing the order of glyphs being allocated.

Solution

• Previously DynamicTextureAtlasBuilder would allocate glyph size + padding, and then copy the glyph into that rectangle - but this means that copying would start at (0,0) meaning we would "blend" with the top and left edges (which breaks things).
• Now we copy to (padding, padding).
• Remove (padding, padding) allocate-able space from the bottom and right to ensure we don't accidentally put textures on the right or bottom edges.

Testing

• Added unit tests to ensure that allocation works and that padding works.
• The rendering change here is quite interesting! https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-23091 Some cases, like the l in black in the Text screenshot look substantially better!

Note this is orthogonal to #23056 which is about TextureAtlasBuilder, not DynamicTextureAtlasBuilder.

[github-actions]

Your PR caused a change in the graphical output of an example or rendering test. This might be intentional, but it could also mean that something broke!
You can review it at https://pixel-eagle.com/project/B04F67C0-C054-4A6F-92EC-F599FEC2FD1D?filter=PR-23091

If it's expected, please add the M-Deliberate-Rendering-Change label.

If this change seems unrelated to your PR, you can consider updating your PR to target the latest main branch, either by rebasing or merging main into it.

[ickshonpe]

Looks good, except I think padding for fonts should probably stay at 2 pixels, at least by default.