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 (https://github.com/aframevr/a-painter/issues/217)
• [x] 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 (https://github.com/aframevr/a-painter/issues/218).
• [ ] Use instances for brushes like spheres or cubes instead of creating a new mesh for each object. (https://github.com/aframevr/a-painter/issues/219)
• [x] 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. (https://github.com/aframevr/a-painter/issues/220)
• [x] Use big buffers and store there the vertices instead of one buffergeometry for each stroke. (https://github.com/aframevr/a-painter/issues/221)