Fix rotated text position when content changes

[mizoz]

Fixes #975.

Rotated text objects were using the current rendered text bounds as their rotation center. When the text content changed size, the center moved, so the rendered object appeared to drift even though its X/Y stayed the same.

This keeps the rotation center stable while rotated text changes content, and applies the same positioning logic to Text, BBText, and BitmapText objects.

Verification:

• npm run build
• npm run check-types
• npx karma start --browsers ChromeHeadlessNoSandbox --single-run --client.mocha.grep="keeps the same rotated origin"
• npx karma start --browsers ChromeHeadlessNoSandbox --single-run --reporters dots

[mizoz]

I checked the failed Semaphore job. The GDJS typing/tests/format jobs passed there; the red jobs are newIDE typing/tests failing while parsing the downloaded newIDE/app/public/libGD.js artifact with TS1128 / SyntaxError: Unexpected token ')'. Travis passed the full PR build, including newIDE tests and Flow.