Performance optimizations

[fernandojsg]

I'll collect here the list of issues related to performance improvement to have an overview vision of what we try to achieve and the status of each task and keep the discussion of each one on its issue instead of flooding this one:

• Fix bounding box issue having a point always on 0,0,0 when using drawRange with on buffer geometries (Fix bounding box issue as each stroke has a point always on 0,0,0 #217)
• Use a texture atlas to pack all the textures from the lines and stamps materials so we don't need to load more than one texture (Texture atlas for stamp and line materials #218).
• Use instances for brushes like spheres or cubes instead of creating a new mesh for each object. (Use instances for brushes that creates geometry #219)
• Use vertex colors instead of setting the material.color so strokes using the same brush but with different colors could be stored on a common buffergeometry. (Use vertex colors instead of material.colors #220)
• Use big buffers and store there the vertices instead of one buffergeometry for each stroke. (Use big buffergeometry buffers instead of one for each stroke #221)