encode kAlpha_8 as grayalpha with sigbits for gray==1

Bug: skia:
Change-Id: Ib61e8e0f62af92d8746f5e73469002e7804a8447
Reviewed-on: https://skia-review.googlesource.com/78481
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Mike Reed <reed@google.com>
diff --git a/src/images/SkImageEncoderFns.h b/src/images/SkImageEncoderFns.h
index d5de065..dd02ebf 100644
--- a/src/images/SkImageEncoderFns.h
+++ b/src/images/SkImageEncoderFns.h
@@ -92,6 +92,18 @@
 }
 
 /**
+ * Transform from kAlpha_8_Config to 2-bytes-per-pixel GrayAlpha.
+ */
+static inline void transform_scanline_A8_to_GrayAlpha(char* SK_RESTRICT dst,
+                                                      const char* SK_RESTRICT src,
+                                                      int width, int, const SkPMColor*) {
+    for (int i = 0; i < width; i++) {
+        *dst++ = 0;         // gray (ignored)
+        *dst++ = *src++;    // alpha
+    }
+}
+
+/**
  * Transform from kRGBA_8888_SkColorType to 3-bytes-per-pixel RGB.
  * Alpha channel data is abandoned.
  */