glsl: Make the tree rebalancer use vector_elements, not components().

glsl: Make the tree rebalancer use vector_elements, not components().

components() includes matrix columns, so if this code encountered a
matrix, it would ask for something like a vec9 or vec16.  This is
clearly not what you want.

Earlier code now prevents this from seeing matrices, but we should still
use vector_elements, for clarity.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>