2021.2/vfx/vfx systemborder dpi fix

[julienamsellem]

Purpose of this PR

Backport of #5955

https://fogbugz.unity3d.com/f/cases/1372213/

Depending on the DPI of the monitor, and the Windows scaling value (125% is the most common trigger), situations arise where the border/title system around VFX graph blocks constantly dirties itself causing a recursive error from the layout engine. This constant spam degrades the performance of the editor.

This change adds specific checks to resolve scenarios where the internal values have been rounded, but the setting values have not. It also adds protection on the event-based call to do a similar check against the old/new values to avoid additional computation.

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Testing status

With a 125% DPI settings on a 4k monitor, I did the following tests:

• staging/2021.2
  • Open a VFX
  • Keep dragging around a context
  • => Error message in the console
• staging/2021.2/2021.2/vfx/vfx-systemborder-dpi-fix
  • Open VFX
  • Keep dragging around a context
  • => No error message in the console

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