Would it be possible to have brushes with either even diameters (for the circle brush), even bases and heights (for the triangle brush) and even side lengths (for the square brushes)?
When you're increasing the size of your brush in-game (either with [/] or the scroll wheel), the sizing would go, "Odd, Even, Odd, Even...".
Use Cases:
Brush sizes are all odd numbers. For example, the diameters of the circle brushes now are: 1, 3, 5, 7, etc.
you can just place two or four times
As K1_2 said, for an even diameter circle you can use an odd size brush 4 times.
If you want a circle of diameter 8, use the size 7 brush 4 times centered on a 2x2 grid
The problem with even size brushes is that the would not be centered on your mouse. There is no center pixel for a circle of diameter 8. The center would be the point in the middle of the 2x2 grid. Same problem for rectangles and triangles.
Oh, I didn't forsee that being a problem with this suggestion.