DRW: Increase weight paint wire depth bias

By default wire would z-fight against the surface.
Increase the bias, also don't adjust the 'w' component
since it causes bias that depends on the view direction.

source/blender/draw/modes/shaders/paint_wire_vert.glsl

@@ -19,7 +19,7 @@ void main()
 	gl_Position = ModelViewProjectionMatrix * vec4(pos, 1.0);
 	/* Add offset in Z to avoid zfighting and render selected wires on top. */
 	/* TODO scale this bias using znear and zfar range. */
-	gl_Position.zw -= exp2(-20.0) * (is_select ? 2.0 : 1.0);
+	gl_Position.z -= (is_select ? 2e-4 : 1e-4);
 
 	if (is_hidden) {
 		gl_Position = vec4(-2.0, -2.0, -2.0, 1.0);