Performance improvements to canvas heatmap implementation

[aashish24]

Currently the heatmap implementation using canvas2D API does not scale and has very poor framerate even with smaller size datasets.

[aashish24]

From @jbeezley

You can apply scale transform to simulate zooming. Yes, it is not the same thing as rerendering, but it makes for an acceptable visual substitute. The behavior that leaflet exhibits is that all animated navigations (zoom and pan) are simulated by CSS transforms. Once the animation is finished the debounced call to rerender the canvas is made.

[aashish24]

@jbeezley some other ideas:

• Render a coarser resolution (either at the data level or at the rendering level) - need to verify if this will really help (this is a very common vis technique).
• Render only when user has stopped panning or zooming. (or combine it with css transformations) like @jbeezley suggested.

[manthey]

Added via PR #574 and #577.