ensure

+ this makes sure that "redirected" programmable attributes are updated using their built-in vertexAttributes, otherwise updateAttributeData would have no effect on locations 0,1,2,3 in programmable GL.
add checks to ofVbo::updateAttributeData

+ makes sure to only allow updates to existing attributes (programmable)
+ makes sure only allocated attributes are allowed updates (all renderers)
ofShader: compute shaders only if supported
Merge branch 'master' of github.com:openframeworks/openFrameworks
Merge pull request #3383 from tgfrerer/fix-updateAttributeData

fix ofVbo::updateAttributeData for programmable GL built-ins
ofVbo: update: remove redundant checks
Merge branch 'master' into refactor-multiwindow