Enable ColorMatrixFilter function chaining

This allows, e.g.:
new ColorMatrixFilter()
    .adjustHue(h)
    .adjustSaturation(s)
    .adjustBrightness(b);
add return value to ColorMatrixFilter.concatValues
fixed that 'Texture.empty' sometimes did not return the requested size (refs #569)
Merge pull request #434 from tconkling/chainableColorMatrixFilter

Enable ColorMatrixFilter function chaining
small indentation fix
enhanced API docs of Button class a little
added 'readjustSize' method, analog to the same method in the Image class (closes #575)
added more API docs to the 'fromTexture' method
moved try/catch around actual instantiation (closes #579)
now dispatching an IO_ERROR when an asset can't be loaded from an URL
removed erroneous call (refs #579)
small simplification
now loading assets in parallel (closes #502)
forgot to clear queue after processing in last commit (refs #502)

Furthermore, aborting queue processing should now work in all cases again;
and you can make several "loadQueue" calls simultaneously now.
small API reference improvement
fixed that 'loadQueue' did not enter progress callback when queue was empty
fixed that blend mode was not reset after drawing into RenderTexture (closes #585)
added check for maximum number of quads
bitmap font composition now uses pools for all temporary objects (closes #583, #493)
added missing reset cleanings in Tween pooling (closes #586)