Update git submodules
- Update extensions/Math from branch 'REL1_39' to 88f35be0cf4d16be9de6f323f04bfbbdfdd696aa
- Remove PNG rendering mode
Removes functionality to- select PNG rendering mode
- automated fallback to PNG images for SVG rendering
- PNG mode related class and tests
However, PNG images received from mathoid / restbase are still stored.
Bug: T311620 Change-Id: I595926027433182cc0396570bc3f1ce0cd2cafb3 (cherry picked from commit 110656335fd703a702591400f0db9fe88a7fa2c0)
- Remove PNG rendering mode