Don't globally set float precision to mediump, only fragment
shaders need that and defining it for vertex shaders causes
tilemap cracks.
Also manually define low precision for variables that hold
color / alpha values.
The drawing is now completely shader based, which makes away
with all usage of the depracted matrix stack. This also allows
us to do things like simple translations and texture coordinate
translation directly instead of doing everything indirectly
through matrices.
Fixed vertex attributes ('vertexPointer()' etc) are also
replaced with user defined attribute arrays.
Jonas Kulla