Brightness example: Fix maximum value for brightness

[bclausdorff]

Issue
The colors in the color brightness example always seem to be rather dark. Even when moving the cursor to the lowest position possible.

Reason
The maximum value for brightness is being set to the width of the canvas. As mouseY is being used as the brightness of the color and the height of the canvas is much lower than the width, it is impossible to reach full brightness for a bar.

Fix
This can be fixed by setting the maximum value for brightness to the height of the canvas.

[REAS]

Thank you!