shaped-texture: Simplify pipeline creation

Split out pipeline creation to a separate function so that we don't
have so much dense code in the paint function itself, and remove some
indentation levels.

Also, don't use our own template for the unmasked pipeline, since it
has nothing different from the default pipeline template.

We also don't store the pipelines anymore since their creation isn't
really helping us; we set the mask texture and paint texture on every
paint anyway.

https://bugzilla.gnome.org/show_bug.cgi?id=706930