chiron: Enable ro.hwui.render_ahead and set it to 20 frames

* Rendering ahead makes some animations handling better (especially qs pulldown on a13 qpr2)

Change-Id: I36f976248e793ee505cd7a9801b14b9f331288d2

vendor.prop

@@ -129,6 +129,7 @@ debug.cpurend.vsync=false
 debug.renderengine.backend=skiaglthreaded
 debug.sf.enable_hwc_vds=1
 debug.sf.hw=1
+ro.hwui.render_ahead=20
 ro.opengles.version=196610
 vendor.display.disable_avr=1
 vendor.display.disable_partial_split=1