Fixes compatibility with emscripten 1.39.5+ .
Most input callbacks now require a target and no longer support NULL
defaults.
This commit changes all required null targets to the expected default in
the binding phase.
Since for canvas-related callbacks there is no default, the "#canvas"
selector is used instead.
Additionally, since canvasX and canvasY event properties are no longer
supported, event positions are computed from "clientX" and "clientY" and
the "#canvas" bounding client rect.
4a50f5ff65
