Use an alpha type that matches the color type in Android software ren…

…dering (flutter#4681)

Fixes flutter/flutter#14709

shell/platform/android/android_surface_software.cc

@@ -18,13 +18,17 @@ namespace shell {
 
 namespace {
 
-bool GetSkColorType(int32_t buffer_format, SkColorType* color_type) {
+bool GetSkColorType(int32_t buffer_format,
+                    SkColorType* color_type,
+                    SkAlphaType* alpha_type) {
   switch (buffer_format) {
     case WINDOW_FORMAT_RGB_565:
       *color_type = kRGB_565_SkColorType;
+      *alpha_type = kOpaque_SkAlphaType;
       return true;
     case WINDOW_FORMAT_RGBA_8888:
       *color_type = kRGBA_8888_SkColorType;
+      *alpha_type = kPremul_SkAlphaType;
       return true;
     default:
       return false;
@@ -33,8 +37,10 @@ bool GetSkColorType(int32_t buffer_format, SkColorType* color_type) {
 
 }  // anonymous namespace
 
-AndroidSurfaceSoftware::AndroidSurfaceSoftware()
-    : AndroidSurface(), target_color_type_(kRGBA_8888_SkColorType) {}
+AndroidSurfaceSoftware::AndroidSurfaceSoftware() {
+  GetSkColorType(WINDOW_FORMAT_RGBA_8888, &target_color_type_,
+                 &target_alpha_type_);
+}
 
 AndroidSurfaceSoftware::~AndroidSurfaceSoftware() = default;
 
@@ -75,7 +81,7 @@ sk_sp<SkSurface> AndroidSurfaceSoftware::AcquireBackingStore(
   }
 
   SkImageInfo image_info = SkImageInfo::Make(
-      size.fWidth, size.fHeight, target_color_type_, kPremul_SkAlphaType);
+      size.fWidth, size.fHeight, target_color_type_, target_alpha_type_);
 
   sk_surface_ = SkSurface::MakeRaster(image_info);
 
@@ -100,10 +106,10 @@ bool AndroidSurfaceSoftware::PresentBackingStore(
   }
 
   SkColorType color_type;
-  if (GetSkColorType(native_buffer.format, &color_type)) {
-    SkImageInfo native_image_info =
-        SkImageInfo::Make(native_buffer.width, native_buffer.height, color_type,
-                          kPremul_SkAlphaType);
+  SkAlphaType alpha_type;
+  if (GetSkColorType(native_buffer.format, &color_type, &alpha_type)) {
+    SkImageInfo native_image_info = SkImageInfo::Make(
+        native_buffer.width, native_buffer.height, color_type, alpha_type);
 
     std::unique_ptr<SkCanvas> canvas = SkCanvas::MakeRasterDirect(
         native_image_info, native_buffer.bits,
@@ -143,7 +149,7 @@ bool AndroidSurfaceSoftware::SetNativeWindow(
   int32_t window_format = ANativeWindow_getFormat(native_window_->handle());
   if (window_format < 0)
     return false;
-  if (!GetSkColorType(window_format, &target_color_type_))
+  if (!GetSkColorType(window_format, &target_color_type_, &target_alpha_type_))
     return false;
   return true;
 }

shell/platform/android/android_surface_software.h

@@ -44,6 +44,7 @@ class AndroidSurfaceSoftware : public AndroidSurface,
 
   fxl::RefPtr<AndroidNativeWindow> native_window_;
   SkColorType target_color_type_;
+  SkAlphaType target_alpha_type_;
 
   FXL_DISALLOW_COPY_AND_ASSIGN(AndroidSurfaceSoftware);
 };