Fix off-thread InvalidatePlot calls (47)

[VisualMelon]

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Fixes various threading issues discovered while investigating #47

• BeginInvoke was backwards
• Not locking when updating/rendering
• Not ensuring Update is called before Render if invalidated (this was the issue causing the exact behaviour observed in #47)

Been a while since I wrote this sort of stuff, so could really do with a second pair of eyes.