README: s/draw pretty pictures/render/
README: s/draw pretty pictures/render/

This tweaks the overview of Cogl to use more technical terminology
instead of saying Cogl is used to "draw pretty pictures".

djdeath/cogl,djdeath/cogl,Distrotech/cogl,gcampax/cogl,gcampax/cogl,djdeath/cogl-android,djdeath/cogl-android,gcampax/cogl,collects/cogl,gcampax/cogl,collects/cogl,Distrotech/cogl,Distrotech/cogl,djdeath/cogl-android,Distrotech/cogl,djdeath/cogl