Added helpers for encoding colors into shader format.
Separate same-for-whole-quad from varies-per-quad-vertex attributes.

We have to copy the same-for-whole-quad data into the vertex buffer four times.
Since it's the same, we can do that more efficiently than writing it out float
by float each time. This commit just rearranges things so that is possible, the
next commit will introduce the bulk copy.
Added GLBuffer.reset and GLBuffer.Float.add(GLBuffer.Float).
Removed GLBuffer.Float.add(int).

Jamming int data into a float buffer for use in a shader turned out to be too
unreliable.
Copy our stable attrs into a buffer; splat that 4x.

This also makes it easier for a derived shader to add additional stable
attributes if desired.