Don't Close Brush Radius Radial Menu with Left Click

On tablet, the brush radius radial menu (activated with F in sculpt mode) is pretty much unusable with tablets without hover (like an iPad with Astropad), because as soon as you try to change the radius with your stylus, it left clicks and closes the radial menu.

Suggest making the radial confirm when left mouse button is lifted up instead of when it is pressed down.

Issue also reported here:

• developer.blender.org/T95628
• blender.stackexchange.com/questions/230689/alternate-methods-to-adjust-brush-size-strength-for-sculpting

Relevant source code:

github.com/blender/blender/blob/0491ba09c21a89f3ac32cb93a7800c95aacb0816/source/blender/windowmanager/intern/wm_operators.c