Match the current Filter Effects spec

This commit is a cherry-pick of 19f7778.

The current draft of the [filter spec](https://dvcs.w3.org/hg/FXTF/raw-file/tip/filters/index.html)
describes brightness, contrast, hue-rotate, and saturate functions that
roughly match our layer's setBrightness, setContrast, setHue, and
setSaturation methods.  These changes make the range of our methods match
the corresponding functions in the spec.  The one exception is the
brightness function.  The spec says it has a range of 0 to positive infinity.
The WebKit implementation accepts a range of -1 to 1 (as of
https://github.com/WebKit/webkit/commit/8f4765e569).  There's an open
(ticket)[https://www.w3.org/Bugs/Public/show_bug.cgi?id=15647] recommending
that the spec be changed to match.

I'm not stuck on having our methods match those of the filter spec, but the
parity would be nice.

These changes leave the WebGL map renderer "broken" (whacky colors).  It would
be straightforward to update the current fragment shader to handle the new
range of hue, but the brightness, contrast, and saturation handling will
need to be reworked.

For inspiration, here are the color transformation
matrix calculations the WebKit filters:
8f4765e569/Source/WebCore/platform/graphics/chromium/cc/CCRenderSurfaceFilters.cpp (L64-80)

Conflicts:

	src/ol/renderer/dom/domlayerrenderer.js

src/ol/layer/layer.js

@@ -40,13 +40,13 @@ ol.layer.Layer = function(layerOptions) {
   this.setBrightness(
       goog.isDef(layerOptions.brightness) ? layerOptions.brightness : 0);
   this.setContrast(
-      goog.isDef(layerOptions.contrast) ? layerOptions.contrast : 0);
+      goog.isDef(layerOptions.contrast) ? layerOptions.contrast : 1);
   this.setHue(
       goog.isDef(layerOptions.hue) ? layerOptions.hue : 0);
   this.setOpacity(
       goog.isDef(layerOptions.opacity) ? layerOptions.opacity : 1);
   this.setSaturation(
-      goog.isDef(layerOptions.saturation) ? layerOptions.saturation : 0);
+      goog.isDef(layerOptions.saturation) ? layerOptions.saturation : 1);
   this.setVisible(
       goog.isDef(layerOptions.visible) ? layerOptions.visible : true);
 
@@ -164,6 +164,23 @@ ol.layer.Layer.prototype.isReady = function() {
 
 
 /**
+ * Adjust the layer brightness.  A value of -1 will render the layer completely
+ * black.  A value of 0 will leave the brightness unchanged.  A value of 1 will
+ * render the layer completely white.  Other values are linear multipliers on
+ * the effect (values are clamped between -1 and 1).
+ *
+ * The filter effects draft [1] says the brightness function is supposed to
+ * render 0 black, 1 unchanged, and all other values as a linear multiplier.
+ *
+ * The current WebKit implementation clamps values between -1 (black) and 1
+ * (white) [2].  There is a bug open to change the filter effect spec [3].
+ *
+ * TODO: revisit this if the spec is still unmodified before we release
+ *
+ * [1] https://dvcs.w3.org/hg/FXTF/raw-file/tip/filters/index.html
+ * [2] https://github.com/WebKit/webkit/commit/8f4765e569
+ * [3] https://www.w3.org/Bugs/Public/show_bug.cgi?id=15647
+ *
  * @param {number} brightness Brightness.
  */
 ol.layer.Layer.prototype.setBrightness = function(brightness) {
@@ -179,10 +196,14 @@ goog.exportProperty(
 
 
 /**
+ * Adjust the layer contrast.  A value of 0 will render the layer completely
+ * grey.  A value of 1 will leave the contrast unchanged.  Other values are
+ * linear multipliers on the effect (and values over 1 are permitted).
+ *
  * @param {number} contrast Contrast.
  */
 ol.layer.Layer.prototype.setContrast = function(contrast) {
-  contrast = goog.math.clamp(contrast, -1, 1);
+  contrast = Math.max(0, contrast);
   if (contrast != this.getContrast()) {
     this.set(ol.layer.LayerProperty.CONTRAST, contrast);
   }
@@ -194,6 +215,8 @@ goog.exportProperty(
 
 
 /**
+ * Apply a hue-rotation to the layer.  A value of 0 will leave the hue
+ * unchanged.  Other values are degrees around the color circle.
  * @param {number} hue Hue.
  */
 ol.layer.Layer.prototype.setHue = function(hue) {
@@ -223,10 +246,15 @@ goog.exportProperty(
 
 
 /**
+ * Adjust layer saturation.  A value of 0 will render the layer completely
+ * unsaturated.  A value of 1 will leave the saturation unchanged.  Other
+ * values are linear multipliers of the effect (and values over 1 are
+ * permitted).
+ *
  * @param {number} saturation Saturation.
  */
 ol.layer.Layer.prototype.setSaturation = function(saturation) {
-  saturation = goog.math.clamp(saturation, -1, 1);
+  saturation = Math.max(0, saturation);
   if (saturation != this.getSaturation()) {
     this.set(ol.layer.LayerProperty.SATURATION, saturation);
   }