Kernel: Make VirtIO GPU buffer flipping more spec compliant

The spec requires a flush after setting the new buffer resource id,
which is required by QEMUs SDL backend but not the GTK backend. This
brings us in line with the spec and makes it work for the SDL backend.

Kernel/Graphics/VirtIOGPU/VirtIOFrameBufferDevice.cpp

@@ -128,6 +128,8 @@ void VirtIOFrameBufferDevice::set_buffer(int buffer_index)
         return;
     m_current_buffer = &buffer;
     m_gpu.set_scanout_resource(m_scanout.value(), buffer.resource_id, display_info().rect);
+    m_gpu.flush_displayed_image(buffer.dirty_rect, buffer.resource_id); // QEMU SDL backend requires this (as per spec)
+    buffer.dirty_rect = {};
 }
 
 int VirtIOFrameBufferDevice::ioctl(FileDescription&, unsigned request, FlatPtr arg)
@@ -196,6 +198,18 @@ int VirtIOFrameBufferDevice::ioctl(FileDescription&, unsigned request, FlatPtr a
                 if (&buffer == m_current_buffer) {
                     // Flushing directly to screen
                     flush_displayed_image(dirty_rect, buffer);
+                    buffer.dirty_rect = {};
+                } else {
+                    if (buffer.dirty_rect.width == 0 || buffer.dirty_rect.height == 0) {
+                        buffer.dirty_rect = dirty_rect;
+                    } else {
+                        auto current_dirty_right = buffer.dirty_rect.x + buffer.dirty_rect.width;
+                        auto current_dirty_bottom = buffer.dirty_rect.y + buffer.dirty_rect.height;
+                        buffer.dirty_rect.x = min(buffer.dirty_rect.x, dirty_rect.x);
+                        buffer.dirty_rect.y = min(buffer.dirty_rect.y, dirty_rect.y);
+                        buffer.dirty_rect.width = max(current_dirty_right, dirty_rect.x + dirty_rect.width) - buffer.dirty_rect.x;
+                        buffer.dirty_rect.height = max(current_dirty_bottom, dirty_rect.y + dirty_rect.height) - buffer.dirty_rect.y;
+                    }
                 }
             }
         }

Kernel/Graphics/VirtIOGPU/VirtIOFrameBufferDevice.h

@@ -19,6 +19,7 @@ class VirtIOFrameBufferDevice final : public BlockDevice {
         size_t framebuffer_offset { 0 };
         u8* framebuffer_data { nullptr };
         VirtIOGPUResourceID resource_id { 0 };
+        VirtIOGPURect dirty_rect {};
     };
 
 public: