Autodesk: testUsdView shots must be UI scale independent

[erikaharrison-adsk]

Description of Change(s)

• Compensate for the device scaling ratio when setting the original window size of the tests.
• Use the original window size in StageView so we avoid problems with integer rounding in the scaling round trip (widgetSize is round(round(originalSize / scale) * scale) so it's not necessarily the same).
• Round up the widget window size so we can truncate it to the original window size when taking a screenshot.

Link to proposal (if applicable)

• N/A

Fixes Issue(s)

• N/A

Checklist

• I have created this PR based on the dev branch

• I have followed the coding conventions

• I have added unit tests that exercise this functionality (Reference:
  testing guidelines)

• I have verified that all unit tests pass with the proposed changes

• I have submitted a signed Contributor License Agreement (Reference:
  Contributor License Agreement instructions)

[jesschimein]

Filed as internal issue #USD-10644

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