MM-49395: Increase compression for previews and emojis (#21985)

We also reduce the max size of uploaded emojis to 512KiB. https://mattermost.atlassian.net/browse/MM-49395 ```release-note Max size of uploaded emojis is now reduced to 512KiB to reduce image download bandwidth. ```
2023-01-05 19:34:22 +05:30
--- a/tests/10000x1_expected_preview.png
+++ b/tests/10000x1_expected_preview.png
--- a/tests/10000x1_expected_thumb.png
+++ b/tests/10000x1_expected_thumb.png
--- a/tests/1x10000_expected_preview.png
+++ b/tests/1x10000_expected_preview.png
--- a/tests/1x10000_expected_thumb.png
+++ b/tests/1x10000_expected_thumb.png
--- a/tests/fill_test_16bit_rgb_out.png
+++ b/tests/fill_test_16bit_rgb_out.png
--- a/tests/fill_test_16bit_rgba_out.png
+++ b/tests/fill_test_16bit_rgba_out.png
--- a/tests/fill_test_8bit_palette_out.png
+++ b/tests/fill_test_8bit_palette_out.png
--- a/tests/fill_test_8bit_rgb_out.png
+++ b/tests/fill_test_8bit_rgb_out.png
--- a/tests/fill_test_8bit_rgba_out.png
+++ b/tests/fill_test_8bit_rgba_out.png