Bug 523950 - Part 3. Add preferences to control animated image decoding behaviour. r=tnikkel

image.animated.decode-on-demand.threshold-kb is the maximum size in kB
that the aggregate frames of an animation can use before it starts to
discard already displayed frames, and redecode them as necessary. The
lower it is set to, the less overall memory we will consume at the
expense of execution time for as long as the tab with the animation(s)
above the threshold are kept open.

image.animated.decode-on-demand.batch-size is the minimum number of
frames we want to have buffered ahead of an animation's currently
displayed frame. The decoding will request this number of frames at a
time to maximize use of memory caching. Note that this is related to the
above preference as well; increasing the batch size will in effect raise
what the minimum threshold. This simplifies the logic in patches later
in the series.

gfx/thebes/gfxPrefs.h

@@ -521,6 +521,8 @@ private:
   DECL_GFX_PREF(Live, "gl.require-hardware",                   RequireHardwareGL, bool, false);
   DECL_GFX_PREF(Live, "gl.use-tls-is-current",                 UseTLSIsCurrent, int32_t, 0);
 
+  DECL_GFX_PREF(Live, "image.animated.decode-on-demand.threshold-kb", ImageAnimatedDecodeOnDemandThresholdKB, uint32_t, 20480);
+  DECL_GFX_PREF(Live, "image.animated.decode-on-demand.batch-size", ImageAnimatedDecodeOnDemandBatchSize, uint32_t, 6);
   DECL_GFX_PREF(Live, "image.cache.factor2.threshold-surfaces", ImageCacheFactor2ThresholdSurfaces, int32_t, -1);
   DECL_GFX_PREF(Once, "image.cache.size",                      ImageCacheSize, int32_t, 5*1024*1024);
   DECL_GFX_PREF(Once, "image.cache.timeweight",                ImageCacheTimeWeight, int32_t, 500);

modules/libpref/init/all.js

@@ -4693,6 +4693,15 @@ pref("toolkit.zoomManager.zoomValues", ".3,.5,.67,.8,.9,1,1.1,1.2,1.33,1.5,1.7,2
 // Image-related prefs
 //
 
+// The maximum size (in kB) that the aggregate frames of an animation can use
+// before it starts to discard already displayed frames and redecode them as
+// necessary.
+pref("image.animated.decode-on-demand.threshold-kb", 20480);
+
+// The minimum number of frames we want to have buffered ahead of an
+// animation's currently displayed frame.
+pref("image.animated.decode-on-demand.batch-size", 6);
+
 // Maximum number of surfaces for an image before entering "factor of 2" mode.
 // This in addition to the number of "native" sizes of an image. A native size
 // is a size for which we can decode a frame without up or downscaling. Most