Bug 365324

I found a fatal flaws in the dynamic brush engine!

If you insert a negative value into the "Mass" variable in the dynamic brush engine (it's at the dynamic settings tab) it will make a brush stroke that will continue happening for eternity and will never stop! you are basically forced to stop the loading of the brush or you wont be able to draw anything.

It also does an infinite line when you put the "Drag" variable in the "Dynamic settings tab" to very high values! (like 49.0 or 50.0 for example)
Krita actually crushed for me because of this! it did not even let me stop the brush process!

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Another improvements that should be made to the Dynamic Brush engine btw:
1. Put sliders to determine the variables in the "dynamic settings" tab, so that variables can be changed more quickly without inputting numbers
2. The size of the brush should be determined by the normal way brush sizes are determined, not with just with the "Diameter" variable 
3. The diameter variable only changes the size of the brush when you use "circle" in the shape tab, but doesn't change the size of it in all the other options!
4. obscure variables: the lines option in the "shape" tab has two variables, and you can see their name only when holding the mouse above them, and the circle option has a checkbox with the word "two", which is rather obscure.)
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