Support reprojection

[katamartin]

Makes a few tweaks to support on-the-fly projection updates (#14)

• Redefines draw method every time projection changes
• Clears the draw buffer on every invalidation
• Avoids silently dropping old regl references on every aspect change
• Makes viewport tracking the explicit responsibility of Regl + Raster
  • Avoids usage of stale context values read in draw calls in Raster