Allow Update and Draw functions to be turned off

allow nil to be passed as an argument to SetUpdateFunction and SetDrawFunction to turn off said functions.
Merge pull request #267 from sigatrev/Update-and-Draw

Allow Update and Draw functions to be turned off
fix ActorFrame diffusing and glowing children

When an ActorFrame's parent has a diffuse or glow, the ActorFrame uses
the static tempState in Actor::PreDraw(); When the ActorFrame draw's
it's children, those children also use tempState because their parent
has a diffuse( making their internalDiffuse not 1,1,1,1 ). If any child
is diffused it'll will change tempState, which will affect the
internalDiffuse given to the next child, and so on.

By determining the diffuse and glow to be passed to the children before
drawing any of them, any changes to tempState will not affect the
diffuse of subsequent children.
Merge pull request #269 from sigatrev/ActorFrame_diffuse

fix ActorFrame diffusing and glowing children
Merge branch 'master' of https://github.com/stepmania/stepmania