WindowServer: Load multiple scaled versions of Bitmaps and Cursors

This enables rendering of mixed-scale screen layouts with e.g. high
resolution cursors and window button icons on high-dpi screens while
using lower resolution bitmaps on regular screens.

Userland/Services/WindowServer/MultiScaleBitmaps.cpp

@@ -0,0 +1,70 @@
+/*
+ * Copyright (c) 2020, the SerenityOS developers.
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include "MultiScaleBitmaps.h"
+#include "Screen.h"
+
+namespace WindowServer {
+
+const Gfx::Bitmap& MultiScaleBitmaps::bitmap(int scale_factor) const
+{
+    auto it = m_bitmaps.find(scale_factor);
+    if (it == m_bitmaps.end()) {
+        it = m_bitmaps.find(1);
+        if (it == m_bitmaps.end())
+            it = m_bitmaps.begin();
+    }
+    // We better found *something*
+    if (it == m_bitmaps.end()) {
+        dbgln("Could not find any bitmap in this MultiScaleBitmaps");
+        VERIFY_NOT_REACHED();
+    }
+    return it->value;
+}
+
+RefPtr<MultiScaleBitmaps> MultiScaleBitmaps::create(StringView const& filename, StringView const& default_filename)
+{
+    auto per_scale_bitmap = adopt_ref(*new MultiScaleBitmaps());
+    if (per_scale_bitmap->load(filename, default_filename))
+        return per_scale_bitmap;
+    return {};
+}
+
+bool MultiScaleBitmaps::load(StringView const& filename, StringView const& default_filename)
+{
+    Optional<Gfx::BitmapFormat> bitmap_format;
+    bool did_load_any = false;
+
+    auto add_bitmap = [&](StringView const& path, int scale_factor) {
+        auto bitmap = Gfx::Bitmap::load_from_file(path, scale_factor);
+        if (bitmap) {
+            auto bitmap_format = bitmap->format();
+            if (m_format == Gfx::BitmapFormat::Invalid || m_format == bitmap_format) {
+                if (m_format == Gfx::BitmapFormat::Invalid)
+                    m_format = bitmap_format;
+
+                did_load_any = true;
+                m_bitmaps.set(scale_factor, bitmap.release_nonnull());
+            } else {
+                dbgln("Bitmap {} (scale {}) has format inconsistent with the other per-scale bitmaps", path, bitmap->scale());
+            }
+        }
+    };
+
+    Screen::for_each_scale_factor_in_use([&](int scale_factor) {
+        add_bitmap(filename, scale_factor);
+        return IterationDecision::Continue;
+    });
+    if (!did_load_any && !default_filename.is_null() && !default_filename.is_empty()) {
+        Screen::for_each_scale_factor_in_use([&](int scale_factor) {
+            add_bitmap(default_filename, scale_factor);
+            return IterationDecision::Continue;
+        });
+    }
+    return did_load_any;
+}
+
+}