Basic shader profiling.

ShadingSystem::attribute ("profile", 1) turns on profiling, which will
attempt to time the total amount of runtime spent actually executing
shaders.  This will be included in the runtime statistics, both as a
total as well as listing the "top 5" individual shader groups that
accounted for the most time.

Profiling isn't free -- it adds a small runtime cost, because it must do
timer queries before and after the main entry point to every shader
invocation, so only turn it on if you are interested in the results.
(That said, it's surprisingly inexpensive -- doesn't seem to throw
runtime off by more than 1%.)

Example output looks like this:

    Execution profile:
      Total shader execution time: 43.32s
      Most expensive shader groups:
        8.08s sky_barry/material
        3.48s sky/material
        2.39s sam/sam_head_bangs_safari.hair
        1.97s flnt/flnt_head_base_def.hair
        1.97s sam_accessories/sam_pony_base_safari.hair