Use appropriate names, and annotate the names with the types — 'u' for
uniforms, 'a' for attributes. The common preambles for shaders are split
from the bodies, so we need some way to distinguish the uniforms and the
attributes just from their name.
For the root node we do not need to use blending, as it does not have
any backdrop to blend into. We can use a simpler 'blit' program that
only takes the content of the source and fills the texture quad with
it.