Fix some of the test-suite timeouts (#882)
handle empty GeoJSON tiles, ref #882
Allow specifying which test-suite tests to run
Fix callback condition order
Fix geojson rendering test timeout (fixes #882)
Fix rendering regressions (fixes #883)
Align vertex attributes at 4 byte boundaries

- Introduces a sprite atlas
- Moves icon rendering to use the sprite atlas
- Unifies icon and glyph buffer structures and vertex shaders
fix background and line images
fix remaining rendering issues