backends: implement custom EGL and renderer initialization

Compositors now have more control over how the backend creates its
renderer. Currently all backends create an EGL/GLES2 renderer, so
the necessary attributes for creating the context are passed to a
user-provided callback function. It is responsible for initializing
provided wlr_egl and to return a renderer. On fail, return 0.

Fixes #987

include/backend/drm/renderer.h

@@ -5,6 +5,7 @@
 #include <gbm.h>
 #include <stdbool.h>
 #include <stdint.h>
+#include <wlr/backend.h>
 #include <wlr/render/wlr_renderer.h>
 
 struct wlr_drm_backend;
@@ -32,7 +33,7 @@ struct wlr_drm_surface {
 };
 
 bool init_drm_renderer(struct wlr_drm_backend *drm,
-	struct wlr_drm_renderer *renderer);
+	struct wlr_drm_renderer *renderer, wlr_renderer_create_func_t create_render);
 void finish_drm_renderer(struct wlr_drm_renderer *renderer);
 
 bool init_drm_surface(struct wlr_drm_surface *surf,

include/wlr/backend.h

@@ -20,12 +20,20 @@ struct wlr_backend {
 	} events;
 };
 
+typedef struct wlr_renderer *(*wlr_renderer_create_func_t)(struct wlr_egl *egl, EGLenum platform,
+	void *remote_display, EGLint *config_attribs, EGLint visual_id);
 /**
  * Automatically initializes the most suitable backend given the environment.
  * Will always return a multibackend. The backend is created but not started.
  * Returns NULL on failure.
+ *
+ * The compositor can request to initialize the backend's renderer by setting
+ * the create_render_func. The callback must initialize the given wlr_egl and
+ * return a valid wlr_renderer, or NULL if it has failed to initiaze it.
+ * Pass NULL as create_renderer_func to use the backend's default renderer.
  */
-struct wlr_backend *wlr_backend_autocreate(struct wl_display *display);
+struct wlr_backend *wlr_backend_autocreate(struct wl_display *display,
+	wlr_renderer_create_func_t create_renderer_func);
 /**
  * Start the backend. This may signal new_input or new_output immediately, but
  * may also wait until the display's event loop begins. Returns false on

include/wlr/backend/drm.h

@@ -14,7 +14,8 @@
  * a DRM backend, other kinds of backends raise SIGABRT).
  */
 struct wlr_backend *wlr_drm_backend_create(struct wl_display *display,
-	struct wlr_session *session, int gpu_fd, struct wlr_backend *parent);
+	struct wlr_session *session, int gpu_fd, struct wlr_backend *parent,
+	wlr_renderer_create_func_t create_renderer_func);
 
 bool wlr_backend_is_drm(struct wlr_backend *backend);
 bool wlr_output_is_drm(struct wlr_output *output);

include/wlr/backend/headless.h

@@ -9,7 +9,8 @@
  * Creates a headless backend. A headless backend has no outputs or inputs by
  * default.
  */
-struct wlr_backend *wlr_headless_backend_create(struct wl_display *display);
+struct wlr_backend *wlr_headless_backend_create(struct wl_display *display,
+	wlr_renderer_create_func_t create_renderer_func);
 /**
  * Create a new headless output backed by an in-memory EGL framebuffer. You can
  * read pixels from this framebuffer via wlr_renderer_read_pixels but it is

include/wlr/backend/wayland.h

@@ -16,7 +16,8 @@
  * to NULL for the default behaviour (WAYLAND_DISPLAY env variable or wayland-0
  * default)
  */
-struct wlr_backend *wlr_wl_backend_create(struct wl_display *display, const char *remote);
+struct wlr_backend *wlr_wl_backend_create(struct wl_display *display, const char *remote,
+	wlr_renderer_create_func_t create_renderer_func);
 
 /**
  * Adds a new output to this backend. You may remove outputs by destroying them.

include/wlr/backend/x11.h

@@ -8,7 +8,7 @@
 #include <wlr/types/wlr_output.h>
 
 struct wlr_backend *wlr_x11_backend_create(struct wl_display *display,
-	const char *x11_display);
+	const char *x11_display, wlr_renderer_create_func_t create_renderer_func);
 struct wlr_output *wlr_x11_output_create(struct wlr_backend *backend);
 
 bool wlr_backend_is_x11(struct wlr_backend *backend);

include/wlr/render/wlr_renderer.h

@@ -3,6 +3,7 @@
 
 #include <stdint.h>
 #include <wayland-server-protocol.h>
+#include <wlr/render/egl.h>
 #include <wlr/render/wlr_texture.h>
 #include <wlr/types/wlr_box.h>
 
@@ -16,6 +17,9 @@ struct wlr_renderer {
 	} events;
 };
 
+struct wlr_renderer *wlr_renderer_autocreate(struct wlr_egl *egl, EGLenum platform,
+	void *remote_display, EGLint *config_attribs, EGLint visual_id);
+
 void wlr_renderer_begin(struct wlr_renderer *r, int width, int height);
 void wlr_renderer_end(struct wlr_renderer *r);
 void wlr_renderer_clear(struct wlr_renderer *r, const float color[static 4]);