Use "discard" in image shader

This leads to better looking Timeline profiles in Chrome's developer tools!
Add global opacity support to WebGL image replay

vectorLayer.setOpacity() now works as expected.
Add "color" support to WebGL image replay

This adds support for layer brightness, contrast, hue, saturation.
Merge pull request #28 from elemoine/webgl-point-shaders

[webgl-point] Add support for changing "colors"