fix: broken OSR transparent option

[codebytere]

Description of Change

Closes #32846.

Traced back to #30778 - we no longer have a transparent option in webPreferences, and so

electron/shell/browser/api/electron_api_web_contents.cc

Line 771 in 28ada6e

options.Get(options::kTransparent, &transparent);

would always return false. As of the above PR, we handle transparency in webPreferences by propagating it to webPreferences in the BrowserWindow ctor:

electron/shell/browser/api/electron_api_browser_window.cc

Lines 56 to 57 in 28ada6e

	web_preferences.SetHidden(options::kBackgroundColor,
	ToRGBAHex(SK_ColorTRANSPARENT));

Therefore, in order to properly instantiate a transparent offscreen window, we need to check for the backgroundColor being set to the hex equivalent of SK_ColorTRANSPARENT and proceed with that value.

Tested with https://gist.github.com/29850810e9c5892587729d3d64a25761.

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Fixed broken transparency option in offscreen window rendering.

[release-clerk]

Release Notes Persisted

Fixed broken transparency option in offscreen window rendering.

[trop]

I have automatically backported this PR to "16-x-y", please check out #33051

[trop]

I have automatically backported this PR to "17-x-y", please check out #33052

[trop]

I have automatically backported this PR to "18-x-y", please check out #33053