MM-36295: Do not convert thumbnails/previews to jpeg for png images (#21230)

We were applying a white background to transparent images and converting them to jpegs. This was to make text be legible behind a black preview background. However, this has led to a poor user experience, as users rarely download the full image but always click on previews. Therefore, we need the previews to remain as pngs. To fix this, we just re-encode them as pngs instead of jpgs. ```release-note NONE ```
2022-10-11 19:04:09 +05:30
--- a/tests/10000x1_expected_preview.jpeg
+++ b/tests/10000x1_expected_preview.jpeg
--- a/tests/10000x1_expected_preview.png
+++ b/tests/10000x1_expected_preview.png
--- a/tests/10000x1_expected_thumb.jpeg
+++ b/tests/10000x1_expected_thumb.jpeg
--- a/tests/10000x1_expected_thumb.png
+++ b/tests/10000x1_expected_thumb.png
--- a/tests/1x10000_expected_preview.jpeg
+++ b/tests/1x10000_expected_preview.jpeg
--- a/tests/1x10000_expected_preview.png
+++ b/tests/1x10000_expected_preview.png
--- a/tests/1x10000_expected_thumb.jpeg
+++ b/tests/1x10000_expected_thumb.jpeg
--- a/tests/1x10000_expected_thumb.png
+++ b/tests/1x10000_expected_thumb.png