glamor: Add glamor_program based poly_text and image_text

glamor: Add glamor_program based poly_text and image_text

Accelerates text painting with GPU-based geometry computation and stippling

v2: Simplify get_glyphs, expand single character variable names to
    more descriptive ones. (Markus Wick)
v3: Rebase against the glamor_prepare_* un-renaming (changes by anholt).

Improves x11perf -f8text by 417.908% +/- 11.0144% (n=10)

Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Eric Anholt <eric@anholt.net>