core: Add GraphicsBufferOptions::software

The software flag indicates whether the graphics buffer allocator needs
to allocate a buffer suitable for software rendering. Its intended usage
is to allow the gbm allocator to allocate both dmabuf and dumb buffers.

src/backends/virtual/virtual_qpainter_backend.cpp

@@ -80,6 +80,7 @@ std::shared_ptr<VirtualQPainterBufferSlot> VirtualQPainterSwapchain::acquire()
     ShmGraphicsBuffer *buffer = m_allocator->allocate(GraphicsBufferOptions{
         .size = m_size,
         .format = m_format,
+        .software = true,
     });
     if (!buffer) {
         qCDebug(KWIN_VIRTUAL) << "Did not get a new Buffer from Shm Pool";

src/backends/wayland/wayland_qpainter_backend.cpp

@@ -84,6 +84,7 @@ std::shared_ptr<WaylandQPainterBufferSlot> WaylandQPainterSwapchain::acquire()
     ShmGraphicsBuffer *buffer = m_allocator->allocate(GraphicsBufferOptions{
         .size = m_size,
         .format = m_format,
+        .software = true,
     });
     if (!buffer) {
         qCDebug(KWIN_WAYLAND_BACKEND) << "Did not get a new Buffer from Shm Pool";

src/backends/x11/windowed/x11_windowed_qpainter_backend.cpp

@@ -91,6 +91,7 @@ std::shared_ptr<X11WindowedQPainterLayerBuffer> X11WindowedQPainterLayerSwapchai
     ShmGraphicsBuffer *graphicsBuffer = m_allocator->allocate(GraphicsBufferOptions{
         .size = m_size,
         .format = m_format,
+        .software = true,
     });
     if (!graphicsBuffer) {
         qCWarning(KWIN_X11WINDOWED) << "Failed to allocate a shared memory graphics buffer";

src/core/gbmgraphicsbufferallocator.cpp

@@ -24,6 +24,10 @@ GbmGraphicsBufferAllocator::~GbmGraphicsBufferAllocator()
 
 GbmGraphicsBuffer *GbmGraphicsBufferAllocator::allocate(const GraphicsBufferOptions &options)
 {
+    if (options.software) {
+        return nullptr;
+    }
+
     if (!options.modifiers.isEmpty() && !(options.modifiers.size() == 1 && options.modifiers.first() == DRM_FORMAT_MOD_INVALID)) {
         gbm_bo *bo = gbm_bo_create_with_modifiers(m_gbmDevice,
                                                   options.size.width(),

src/core/graphicsbufferallocator.h

@@ -29,6 +29,9 @@ struct GraphicsBufferOptions
 
     /// An optional list of modifiers, see DRM_FORMAT_MOD_*.
     QVector<uint64_t> modifiers;
+
+    /// Whether the graphics buffer should be suitable for software rendering.
+    bool software = false;
 };
 
 class KWIN_EXPORT GraphicsBufferAllocator

src/core/shmgraphicsbufferallocator.cpp

@@ -38,6 +38,9 @@ const ShmAttributes *ShmGraphicsBuffer::shmAttributes() const
 
 ShmGraphicsBuffer *ShmGraphicsBufferAllocator::allocate(const GraphicsBufferOptions &options)
 {
+    if (!options.software) {
+        return nullptr;
+    }
     if (!options.modifiers.isEmpty()) {
         return nullptr;
     }