Agata Cacko authored Mar 30, 2021 and Halla Rempt committed Mar 30, 2021

Before this commit, MyPaint brushes would crash on big sizes
(easy test: size 1000px on blend+paint) because it was
initialized in the stack instead of on the heap.
(For future readers, the wrong/stack initialization for big sizes:
int array[size];
heap initialization:
int array = new array[size]; // and remember to delete [] array;!)

This commit not only removes that initialization which fixes the crash,
but also changes it to use a fixed device instead, which should
help with memory fragmentization and make the brush engine
faster on Windows (hopefully - no benchmarks were done).

BUG:432754