Bug 1551088 - Part 2. Add unpack methods to convert from RGB to RGBX/BGRX. r=lsalzman

Some image decoders (e.g. PNG) may have a native representation of the
data as RGB, and do not have accelerated methods to transform from RGB
to RGBX/BGRX. Exposing this as part of the swizzle/premultiply methods
allows us to write accelerated versions ourselves in a later patch in
this series.

Differential Revision: https://phabricator.services.mozilla.com/D46445

--HG--
extra : moz-landing-system : lando

gfx/2d/Swizzle.cpp

@@ -858,6 +858,31 @@ static void PackToA8(const uint8_t* aSrc, int32_t aSrcGap, uint8_t* aDst,
   PACK_ALPHA_CASE(SurfaceFormat::R8G8B8A8, aDstFormat, aPackFunc) \
   PACK_ALPHA_CASE(SurfaceFormat::A8R8G8B8, aDstFormat, aPackFunc)
 
+template <bool aSwapRB>
+static void UnpackRowRGB24ToRGBA(const uint8_t* aSrc, uint8_t* aDst, int32_t aLength) {
+  // Because we are expanding, we can only process the data back to front in
+  // case we are performing this in place.
+  const uint8_t* src = aSrc + 3 * (aLength - 1);
+  uint8_t* dst = aDst + 4 * (aLength - 1);
+  while (src >= aSrc) {
+    uint8_t r = src[aSwapRB ? 2 : 0];
+    uint8_t g = src[1];
+    uint8_t b = src[aSwapRB ? 0 : 2];
+
+    dst[0] = r;
+    dst[1] = g;
+    dst[2] = b;
+    dst[3] = 0xFF;
+
+    src -= 3;
+    dst -= 4;
+  }
+}
+
+#define UNPACK_ROW_RGB(aDstFormat)                   \
+  FORMAT_CASE_ROW(SurfaceFormat::R8G8B8, aDstFormat, \
+                  UnpackRowRGB24ToRGBA<ShouldSwapRB(SurfaceFormat::R8G8B8, aDstFormat)>)
+
 bool SwizzleData(const uint8_t* aSrc, int32_t aSrcStride,
                  SurfaceFormat aSrcFormat, uint8_t* aDst, int32_t aDstStride,
                  SurfaceFormat aDstFormat, const IntSize& aSize) {
@@ -1007,6 +1032,11 @@ SwizzleRowFn SwizzleRow(SurfaceFormat aSrcFormat, SurfaceFormat aDstFormat) {
     SWIZZLE_ROW_OPAQUE(SurfaceFormat::R8G8B8A8, SurfaceFormat::R8G8B8X8)
     SWIZZLE_ROW_OPAQUE(SurfaceFormat::R8G8B8X8, SurfaceFormat::R8G8B8A8)
 
+    UNPACK_ROW_RGB(SurfaceFormat::R8G8B8X8)
+    UNPACK_ROW_RGB(SurfaceFormat::R8G8B8A8)
+    UNPACK_ROW_RGB(SurfaceFormat::B8G8R8X8)
+    UNPACK_ROW_RGB(SurfaceFormat::B8G8R8A8)
+
     default:
       break;
   }