PixelPaint: Add a general-purpose parallel image processing pipeline

The ImageProcessor singleton is intended to be used by all sorts of
image processing which might take some time to complete; or other
background actions. We're not using BackgroundTask here because this
system is specifically designed to work with task queues and PixelPaint
interaction; e.g. it provides common image processing tasks such as
filter application.

Userland/Applications/PixelPaint/ImageProcessor.cpp

@@ -0,0 +1,69 @@
+/*
+ * Copyright (c) 2022, kleines Filmröllchen <filmroellchen@serenityos.org>.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "ImageProcessor.h"
+
+namespace PixelPaint {
+
+FilterApplicationCommand::FilterApplicationCommand(NonnullRefPtr<Filter> filter, NonnullRefPtr<Layer> target_layer)
+    : m_filter(move(filter))
+    , m_target_layer(move(target_layer))
+{
+}
+
+void FilterApplicationCommand::execute()
+{
+    m_filter->apply(m_target_layer->content_bitmap(), m_target_layer->content_bitmap());
+    m_filter->m_editor->gui_event_loop().deferred_invoke([strong_this = NonnullRefPtr(*this)]() {
+        // HACK: we can't tell strong_this to not be const
+        (*const_cast<NonnullRefPtr<Layer>*>(&strong_this->m_target_layer))->did_modify_bitmap(strong_this->m_target_layer->rect());
+        strong_this->m_filter->m_editor->did_complete_action();
+    });
+}
+
+static Singleton<ImageProcessor> s_image_processor;
+
+ImageProcessor::ImageProcessor()
+    : m_command_queue(MUST(Queue::try_create()))
+    , m_processor_thread(Threading::Thread::construct([this]() {
+        while (true) {
+            if (auto next_command = m_command_queue.try_dequeue(); !next_command.is_error()) {
+                next_command.value()->execute();
+            } else {
+                Threading::MutexLocker locker { m_wakeup_mutex };
+                m_wakeup_variable.wait_while([this]() { return m_command_queue.weak_used() == 0; });
+            }
+        }
+        return 0;
+    },
+          "Image Processor"sv))
+    , m_wakeup_variable(m_wakeup_mutex)
+{
+}
+
+ImageProcessor* ImageProcessor::the()
+{
+    return s_image_processor;
+}
+
+ErrorOr<void> ImageProcessor::enqueue_command(NonnullRefPtr<ImageProcessingCommand> command)
+{
+    if (auto queue_status = m_command_queue.try_enqueue(move(command)); queue_status.is_error())
+        return ENOSPC;
+
+    if (!m_processor_thread->is_started()) {
+        m_processor_thread->start();
+        m_processor_thread->detach();
+    }
+
+    m_wakeup_mutex.lock();
+    m_wakeup_variable.signal();
+    m_wakeup_mutex.unlock();
+
+    return {};
+}
+
+}