v2.x: Allow drawing filled concave polygons

[danngreen]

LVGL will go into an infinite loop if we try to draw a filled concave polygon. The current workaround is to check the polygon for concavity, and draw the outline instead of filling it if it's concave.

Unfortunately a convex polygon can become concave when it's scaled down and the points are rounded to the nearest integer pixel coordinate: it was a rounded rectangle with radius 2 corners that first alerted this to me.

Some ideas for solutions:

• Use ThorVG or look into another polygon renderer
• Split concave polygons into smaller polygons until none of them are concave, then draw them separately