fate: Add webp alpha test

tests/fate/image.mak

@@ -223,8 +223,15 @@ FATE_TIFF-$(call DEMDEC, IMAGE2, TIFF) += $(FATE_TIFF)
 FATE_SAMPLES_AVCONV += $(FATE_TIFF-yes)
 fate-tiff: $(FATE_TIFF-yes)
 
-FATE_SAMPLES_AVCONV-$(call DEMDEC, IMAGE2, WEBP) += fate-webp
-fate-webp: CMD = framecrc -i $(TARGET_SAMPLES)/webp/image_small.webp
+FATE_WEBP += fate-webp-yuv420p
+fate-webp-yuv420p: CMD = framecrc -i $(TARGET_SAMPLES)/webp/image_small.webp
+
+FATE_WEBP += fate-webp-yuva420p
+fate-webp-yuva420p: CMD = framecrc -i $(TARGET_SAMPLES)/webp/1_webp_a.webp
+
+FATE_WEBP-$(call DEMDEC, IMAGE2, WEBP) += $(FATE_WEBP)
+FATE_SAMPLES_AVCONV += $(FATE_WEBP-yes)
+fate-webp: $(FATE_WEBP-yes)
 
 FATE_XBM += fate-xbm10
 fate-xbm10: CMD = framecrc -i $(TARGET_SAMPLES)/xbm/xl.xbm

tests/ref/fate/webp-yuva420p

@@ -0,0 +1,2 @@
+#tb 0: 1/25
+0,          0,          0,        1,   301200, 0x12b071a0