2d/pixel perfect improvements

[unity-cchu]

Please read the Contributing guide before making a PR.

• Read the Graphics repository & Yamato FAQ.

Checklist for PR maker

• Have you updated the changelog? Each package has a CHANGELOG.md file.
• Have you updated or added the documentation for your PR? When you add a new feature, change a property name, or change the behavior of a feature, it's best practice to include related documentation changes in the same PR. If you do add documentation, make sure to add the relevant Graphics Docs team member as a reviewer of the PR. If you are not sure which person to add, see the Docs team contacts sheet.

---

Purpose of this PR

To improve the usability of Pixel Perfect. This does not add new functionality but rather improves existing functionality.

---

Testing status

I have run it through automated test scenes setup and made sure they upgrade properly.

[BoonKiangTan]

Fixes verified on branch:

Version: 2021.2.0a10.1508
Revision: trunk b8c2bb7e8b36
Built: Tue, 16 Mar 2021 16:54:50 GMT
Package branch: 2d/Pixel-Perfect-Improvements (4b8a974)

• Verified Camera Projection and the Camera Size properties are disabled in Camera component if URP Pixel Perfect Camera component is added (Verified warning box appears to inform users)
• Verified Pixel Perfect Camera error message in inspector if no SRP/non-2D URP asset is added to Project Settings > Graphics
• Verified “Windowbox” Crop Frame mode visual artifacts are fixed (Black bars appear around camera view when Windowbox is selected)
• “Letterbox” Crop Frame issue, and by extension, Windowbox and Stretchfill resolution issues are fixed

[phi-lira]

LGTM. @TheoWong-pixel could review UX.

[TheoWong-pixel]

Want to confirm: that it is a warning box or Info box? I believe in @Xiangtingsu's design it is supposed to be an info box which i agree with.

can we also get some images to confirm the disabling please?

[unity-cchu]