Move platform backends to backends directory

This improves file organization in kwin by putting backends in a single
directory.

It also makes easier to discover kwin's low level components for new
contributors because the plugins directory may come as the last place to
look for. When one hears "plugin", the first thing that comes to mind is
regular plugins, not low level backends.

autotests/CMakeLists.txt

@@ -267,7 +267,7 @@ set(testOpenGLContextAttributeBuilder_SRCS
 )
 
 if (HAVE_EPOXY_GLX)
-    set(testOpenGLContextAttributeBuilder_SRCS ${testOpenGLContextAttributeBuilder_SRCS} ../src/plugins/platforms/x11/standalone/glx_context_attribute_builder.cpp)
+    set(testOpenGLContextAttributeBuilder_SRCS ${testOpenGLContextAttributeBuilder_SRCS} ../src/backends/x11/standalone/glx_context_attribute_builder.cpp)
 endif()
 add_executable(testOpenGLContextAttributeBuilder ${testOpenGLContextAttributeBuilder_SRCS})
 target_link_libraries(testOpenGLContextAttributeBuilder Qt::Test)

autotests/opengl_context_attribute_builder_test.cpp

@@ -13,7 +13,7 @@
 
 #include <kwinconfig.h>
 #if HAVE_EPOXY_GLX
-#include "../src/plugins/platforms/x11/standalone/glx_context_attribute_builder.h"
+#include "../src/backends/x11/standalone/glx_context_attribute_builder.h"
 #include <epoxy/glx.h>
 
 #ifndef GLX_GENERATE_RESET_ON_VIDEO_MEMORY_PURGE_NV

src/backends/CMakeLists.txt

@@ -1,2 +1,9 @@
+add_subdirectory(drm)
 add_subdirectory(fakeinput)
+if (HAVE_LINUX_FB_H)
+    add_subdirectory(fbdev)
+endif()
 add_subdirectory(libinput)
+add_subdirectory(virtual)
+add_subdirectory(wayland)
+add_subdirectory(x11)