Memory leaking when (re-)drawing text

[d-buchmann]

Steps to reproduce

1. Run ExampleBrowserGtk3
2. Open "PlotControllerExamples/Show tracker without clicking"
3. Move mouse to update the Tracker and observe the memory allocation (in taskmgr or top)

Platform: Window / Linux
.NET version: Core 3.1

Expected behaviour

Memory usage should basically stay constant.

Actual behaviour

Memory usage increases constantly.

This looks much like a memory leak in the drawing routines.
I let the DrawText / MeasureText methods in GraphicsRenderContext return immediately without drawing, then there was no more leakage to be observed.
On Linux (raspberry pi) the GUI crashed almost instantly when moving the mouse over the plot so it might also be some kind of race condition involved.
Supposedly the memory is leaking everytime text is drawn, but it is only observed when doing the redrawing very frequently.

[d-buchmann]

I managed to mitigate the symptoms by constructing a persistent Pango.Context from the Cairo.Context in SetGraphicsTarget and then constructing a (non-static) Pango.Layout from that in DrawText / MeasureText.
I think i've just scaled the issue down by the factor of how many texts are displayed in the plot simultaneously...maybe some GTK expert can do it properly.

On the Raspberry Pi the behaviour didn't change, but I tested all the other examples without any crash, so I think it's a perfomance issue which might as well be solved by rewriting the example (I don't think it's strictly necessary).

I will try to write up a patch tomorrow.
Still someone would need to check if this interferes with other dependecy versions than those I stated in PR #10.