* Closes https://github.com/emilk/egui/issues/4976
* Part of #4378
* Implements parts of #843
### Background
Some widgets (like `Grid` and `Table`) needs to know the width of future
elements in order to properly size themselves. For instance, the width
of the first column of a grid may not be known until all rows of the
grid has been added, at which point it is too late. Therefore these
widgets store sizes from the previous frame. This leads to "first-frame
jitter", were the content is placed in the wrong place for one frame,
before being accurately laid out in subsequent frames.
### What
This PR adds the function `ctx.request_discard` which discards the
visual output and does another _pass_, i.e. calls the whole app UI code
once again (in eframe this means calling `App::update` again). This will
thus discard the shapes produced by the wrongly placed widgets, and
replace it with new shapes. Note that only the visual output is
discarded - all other output events are accumulated.
Calling `ctx.request_discard` should only be done in very rare
circumstances, e.g. when a `Grid` is first shown. Calling it every frame
will mean the UI code will become unnecessarily slow.
Two safe-guards are in place:
* `Options::max_passes` is by default 2, meaning egui will never do more
than 2 passes even if `request_discard` is called on every pass
* If multiple passes is done for multiple frames in a row, a warning
will be printed on the screen in debug builds:
### Breaking changes
A bunch of things that had "frame" in the name now has "pass" in them
instead:
* Functions called `begin_frame` and `end_frame` are now called
`begin_pass` and `end_pass`
* `FrameState` is now `PassState`
* etc
### TODO
* [x] Figure out good names for everything (`ctx.request_discard`)
* [x] Add API to query if we're gonna repeat this frame (to early-out
from expensive rendering)
* [x] Clear up naming confusion (pass vs frame) e.g. for `FrameState`
* [x] Figure out when to call this
* [x] Show warning on screen when there are several frames in a row with
multiple passes
* [x] Document
* [x] Default on or off?
* [x] Change `Context::frame_nr` name/docs
* [x] Rename `Context::begin_frame/end_frame` and deprecate the old ones
* [x] Test with Rerun
* [x] Document breaking changes
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