use 4-byte aligned vertex attributes for performance reason

reverts 9d5f02ccaa051a0f3459f6fc94df807338fa2552 and 4c0b4a79017bc7c7ae864e6f961f186605be617f
depend on mbgl-core instead of -standalone so we don't have to run the packaging script every time

(this speeds up the build slightly)
add debug line for showing the sprite atlas
[performance] remove glFlush()
fix rendering errors
warp in MBGL_CHECK_ERROR() call
provide explicit constructor to appease GCC
use correct enum