Use the new VAAPI dummy image support for kmsgrab too

src/platform/linux/kmsgrab.cpp

@@ -874,6 +874,8 @@ namespace platf {
       alloc_img() override {
         auto img = std::make_shared<egl::img_descriptor_t>();
 
+        img->width = width;
+        img->height = height;
         img->serial = std::numeric_limits<decltype(img->serial)>::max();
         img->data = nullptr;
         img->pixel_pitch = 4;
@@ -886,16 +888,8 @@ namespace platf {
 
       int
       dummy_img(platf::img_t *img) override {
-        // TODO: stop cheating and give black image
-        if (!img) {
-          return -1;
-        };
-        auto pull_dummy_img_callback = [&img](std::shared_ptr<platf::img_t> &img_out) -> bool {
-          img_out = img->shared_from_this();
-          return true;
-        };
-        std::shared_ptr<platf::img_t> img_out;
-        return snapshot(pull_dummy_img_callback, img_out, 1s, false) != platf::capture_e::ok;
+        // Empty images are recognized as dummies by the zero sequence number
+        return 0;
       }
 
       capture_e
@@ -988,12 +982,10 @@ namespace platf {
           return -1;
         }
 
-        sequence = 0;
-
         return 0;
       }
 
-      std::uint64_t sequence;
+      std::uint64_t sequence {};
     };
 
   }  // namespace kms