Cherry pick #6928 #6932 #6948 #7014

[boundsj]

cc @1ec5

[mention-bot]

@boundsj, thanks for your PR! By analyzing the history of the files in this pull request, we identified @jfirebaugh, @brunoabinader and @kkaefer to be potential reviewers.

[boundsj]

Along with several Qt/Linux related commits that get the build passing in CI again, this brings in:

• [core] Separate pathway for notifying workers of new images/glyphs aa08796
• [core] symbol layer - recalculate style on icon/text size changes 39b0bc0

This does not include:

• [core] separate Backend from View for headless rendering 5cc390d
• [core] move gl::Context to Backend and refactor View a4d259c
• [core] don't load tiles from sources that aren't used 9127299
• [core] change std::array<uint16_t, 2> to mbgl::Size 62b56b7
• [core] correctly set the framebuffer size on retina screens f7801e5

I think this is the correct of action but I wanted to check since these commits that I did not include were brought into the Android release in #7021.

cc @1ec5 @jfirebaugh @ivovandongen

[jfirebaugh]

@boundsj Those look like appropriate changes to cherry pick. I would add #7025 to the list as well.

#7021 pulled a broader set of commits than I would have recommended.

[1ec5]

Note that this PR also pulled in #6899 (but out of order and without 6c30105).