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

render/wlr_renderer.c

@@ -4,6 +4,7 @@
 #include <wlr/render/interface.h>
 #include <wlr/render/wlr_renderer.h>
 #include <wlr/types/wlr_matrix.h>
+#include <wlr/render/gles2.h>
 #include <wlr/util/log.h>
 #include "util/signal.h"
 
@@ -179,3 +180,19 @@ void wlr_renderer_init_wl_shm(struct wlr_renderer *r,
 		}
 	}
 }
+
+struct wlr_renderer *wlr_renderer_autocreate(struct wlr_egl *egl,
+		EGLenum platform, void *remote_display, EGLint *config_attribs, EGLint visual_id) {
+
+	if (!wlr_egl_init(egl, platform, remote_display, config_attribs, visual_id)) {
+		wlr_log(L_ERROR, "Could not initialize EGL");
+		return NULL;
+	}
+
+	struct wlr_renderer *renderer = wlr_gles2_renderer_create(egl);
+	if (!renderer) {
+		wlr_egl_finish(egl);
+	}
+
+	return renderer;
+}