brushlib: allow pressure > 1.0
brushlib: allow pressure > 1.0

Experimental. This allows the brush to continue the pressure response
when using the "pressure gain" brush setting. Which may or may not be
desireable(?)

b3sigma/libmypaint,achadwick/libmypaint,b3sigma/libmypaint,b3sigma/libmypaint,achadwick/libmypaint,achadwick/libmypaint,achadwick/libmypaint