Introduce a 'fast' alpha mode

.. where only 2 filtering modes are potentially
tried, instead of all of them. This is fast than the exhaustive 'best'
mode, and not much worse.

Options for cwebp are:
 -alpha_filter none
 -alpha_filter fast      (<- default)
 -alpha_filter best      (<- slow)

Change-Id: I8cb90ee11b8f981811e013ea4ad5bf72ba3ea7d4

src/webp/encode.h

@@ -75,8 +75,7 @@ typedef struct {
                           // 1 = backward reference counts encoded with
                           // arithmetic encoder). Default is 1.
   int alpha_filtering;    // Predictive filtering method for alpha plane.
-                          // (0 = none, 1 = horizontal, 2 = vertical, 3 = grad,
-                          // 4 = Paeth and 5 = Best of (0 .. 4).
+                          //  0: none, 1: fast, 2: best. Default if 1.
   int alpha_quality;      // Between 0 (smallest size) and 100 (lossless).
                           // Default is 100.
 } WebPConfig;