compositor: Set EGL_PLATFORM env variable for each backend.

I may have missed something, but - since the Wayland compositor
already picks a platform backend, opens a connection and initializes the
backend specific display data structure it doesn't make sense
to let egl pick a platform. If it picks a different one the
display specific data structure will most likely not match.
Thus determine the platform in the Wayland rendering backend by setting
the EGL_PLATFORM env variable.
For the client any other platform than 'wayland' doesn't seem to make
sense.
I'm not sure if I've got the the platform ofr openfwd right.

Signed-off-by: Egbert Eich <eich@freedesktop.org>

compositor/compositor-drm.c

@@ -269,6 +269,7 @@ init_egl(struct drm_compositor *ec, struct udev_device *device)
 		return -1;
 	}
 
+	setenv("EGL_PLATFORM", "drm", 1);
 	ec->drm.fd = fd;
 	ec->base.display = eglGetDisplay(FD_TO_EGL_NATIVE_DPY(ec->drm.fd));
 	if (ec->base.display == NULL) {

compositor/compositor-openwfd.c

@@ -118,6 +118,7 @@ init_egl(struct wfd_compositor *ec)
 		return -1;
 
 	ec->wfd_fd = fd;
+	setenv("EGL_PLATFORM", "drm", 1);
 	ec->base.display = eglGetDisplay(FD_TO_EGL_NATIVE_DPY(ec->wfd_fd));
 	if (ec->base.display == NULL) {
 		fprintf(stderr, "failed to create display\n");

compositor/compositor-wayland.c

@@ -111,6 +111,7 @@ wayland_compositor_init_egl(struct wayland_compositor *c)
 		EGL_NONE
 	};
 
+	setenv("EGL_PLATFORM", "wayland", 1);
 	c->base.display = eglGetDisplay(c->parent.display);
 	if (c->base.display == NULL) {
 		fprintf(stderr, "failed to create display\n");

compositor/compositor-x11.c

@@ -113,6 +113,7 @@ x11_compositor_init_egl(struct x11_compositor *c)
 		EGL_NONE
 	};
 
+	setenv("EGL_PLATFORM", "x11", 1);
 	c->base.display = eglGetDisplay(c->dpy);
 	if (c->base.display == NULL) {
 		fprintf(stderr, "failed to create display\n");