explicitly bump legacy genID on gpu surface

src/image/SkSurface_Gpu.cpp

Index: src/image/SkSurface_Gpu.cpp
diff --git a/src/image/SkSurface_Gpu.cpp b/src/image/SkSurface_Gpu.cpp
index 70866d3445e40bdcf18246b12740f743817b9b0e..3a306ad3e6a882701be4b916e4dc8fe38cc81b35 100644
--- a/src/image/SkSurface_Gpu.cpp
+++ b/src/image/SkSurface_Gpu.cpp
@@ -35,6 +35,8 @@ GrBackendObject SkSurface_Gpu::onGetTextureHandle(TextureHandleAccess access) {
         case kDiscardWrite_TextureHandleAccess:
             // for now we don't special-case on Discard, but we may in the future.
             this->notifyContentWillChange(kRetain_ContentChangeMode);
+            // legacy: need to dirty the bitmap's genID in our device (curse it)
+            fDevice->fLegacyBitmap.notifyPixelsChanged();
             break;
     }
     rt->prepareForExternalIO();