Separator() needs to scale with DPI

[lailoken]

Version/Branch of Dear ImGui:

latest

Back-ends:

N/A

Compiler, OS:

N/A

Full config/build information:

No response

Details:

Separator() needs to scale correctly to be legible in high DPI environments.

Screenshots/Video:

No response

Minimal, Complete and Verifiable Example code:

[ocornut]

Adding notes:
Separator() has been maintaining a legacy hack (9534ef9) because back then I noticed it would break code that people often copied to measure size. Though honestly we should rework it now.
If we make Separator() intervene with layout user needs to know their size anyway.
I'll work on that. It'll also fix #2657

[ocornut]

Fixed with ef022c5 which is advertised as a small breaking change.