The position of the text rendered by `showText` is affected
incrementally by the preceding `showText` operations "on the same line".
For this reason, we keep track of all of them (with their dependencies)
in `sameLineText`.
`sameLineText` can be reset whenever we explicitly position the text
somewhere else. We were previously only doing it for `moveText`, and
this patch updates the code to also do it on `setTextMatrix` (which
resets `this.current.x/y` in `CanvasGraphics` to 0).
The complexity of subsequent `sameLineText` operations dependency
tracking grows quadratically with the number of operations on the same
line, so this patch fixes the performance problem when there are whole
pages of text that only use `setTextMatrix` and not `moveText`.
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