VSE: bilinear upscaling no longer adds transparent border around the image

Part of overall "improve image filtering situation" (#116980), this PR addresses
two issues:
- Bilinear (default) image filtering makes half a source pixel wide transparent
  border around the image. This is very noticeable when scaling images/movies up
  in VSE. However, when there is no scaling up but you have slightly rotated
  image, this creates a "somewhat nice" anti-aliasing around the edge.
- The other filtering kinds (e.g. cubic) do not have this behavior. So they do
  not create unexpected transparency when scaling up (yay), however for slightly
  rotated images the edge is "jagged" (oh no).

More detail and images in PR.

Pull Request: https://projects.blender.org/blender/blender/pulls/117717

tests/python/sequencer_render_tests.py

@@ -50,6 +50,9 @@ def main():
     from modules import render_report
     report = render_report.Report("Sequencer", output_dir, oiiotool)
     report.set_pixelated(True)
+    # default error tolerances are quite large, lower them
+    report.set_fail_threshold(1.0 / 255.0)
+    report.set_fail_percent(0.01)
     report.set_reference_dir("reference")
 
     test_dir_name = Path(test_dir).name