Renames GlrCommand to GlrBatch
Adds a GlrContext object as argument to the constructor of GlrCanvas
Cleans up a bit GlrCanvas code and headers
Removes texture cache from public headers
Defers all GL calls to an internal thread in GlrContext

This is a substantial change in the architecture to allow GlrCanvas to be
thread-safe and GlrLayer to be really data-parallel (not thread-safe).

Also adds a glr-priv.h header to centralize all internal symbols.
Updates git ignore file