Fix MakeRenderContextCurrent to handle software rendering (#6827)

Author: jason-simmons

shell/common/rasterizer.cc

@@ -93,16 +93,16 @@ sk_sp<SkImage> Rasterizer::MakeRasterSnapshot(sk_sp<SkPicture> picture,
                                               SkISize picture_size) {
   TRACE_EVENT0("flutter", __FUNCTION__);
 
-  if (!surface_->MakeRenderContextCurrent()) {
-    return nullptr;
-  }
-
   sk_sp<SkSurface> surface;
   if (surface_ == nullptr || surface_->GetContext() == nullptr) {
     // Raster surface is fine if there is no on screen surface. This might
     // happen in case of software rendering.
     surface = SkSurface::MakeRaster(SkImageInfo::MakeN32Premul(picture_size));
   } else {
+    if (!surface_->MakeRenderContextCurrent()) {
+      return nullptr;
+    }
+
     // When there is an on screen surface, we need a render target SkSurface
     // because we want to access texture backed images.
     surface = SkSurface::MakeRenderTarget(