This commit fixes a number of missing cleanup steps in the unit tests
that kept state alive for longer than necessary:
- the loading tasks were not all being destroyed;
- the find controllers were not being reset;
- the state set in `beforeAll`/`beforeEach` was not all being nulled in
the correspoding `afterAll`/`afterEach` blocks.
Combined this resulted in a steady increase in memory usage of the test
process as the tests ran, climbing up to ~1.5 GB. After this patch the
memory usage remains stable at ~800 MB.
It'll help to make math equations "visible" for screen readers.
MS Office has a specific way to add some MathML code to struc tree leaf
and this patch handles it.
The idea is to insert a span in the text layer with an aria-role set to img
and use the bounding box provided by the attribute field in the tag dict in
order to have non-null dimensions for the image to make it "visible".
When a PDF is "marked" we now generate a separate DOM that represents
the structure tree from the PDF. This DOM is inserted into the <canvas>
element and allows screen readers to walk the tree and have more
information about headings, images, links, etc. To link the structure
tree DOM (which is empty) to the text layer aria-owns is used. This
required modifying the text layer creation so that marked items are
now tracked.
