Function-local `static constexpr` variables can be `constexpr`. This
can reduce memory consumption, binary size, and offer additional
compiler optimizations.
This implements the rotate cw/ccw actions in PDFViewer.
Since the rendered pages are stored in a HashMap for caching,
the bitmap is wrapped in a struct with the current rotation.
This way the caching works as expected while zooming, and a new bitmap
is rendered when the page is rotated.
When holding ctrl and scrolling, the page will be zoomed in an out.
When zoomed in on a page, scrolling with move vertically up and down
the page (or horizontally if shift is being held).
In order to speed up zooming, zoomed bitmaps are cached per-page at
each distinct zoom level. This cache is cleared every 30 seconds to
prevent OOM problems.
