render/gles2: require GL_EXT_unpack_subimage

We implicitly depended on this extension.

render/gles2/renderer.c

@@ -893,6 +893,11 @@ struct wlr_renderer *wlr_gles2_renderer_create(struct wlr_egl *egl) {
 		free(renderer);
 		return NULL;
 	}
+	if (!check_gl_ext(exts_str, "GL_EXT_unpack_subimage")) {
+		wlr_log(WLR_ERROR, "GL_EXT_unpack_subimage not supported");
+		free(renderer);
+		return NULL;
+	}
 
 	renderer->exts.read_format_bgra_ext =
 		check_gl_ext(exts_str, "GL_EXT_read_format_bgra");

render/gles2/texture.c

@@ -55,7 +55,6 @@ static bool gles2_texture_write_pixels(struct wlr_texture *wlr_texture,
 		get_gles2_format_from_wl(texture->wl_format);
 	assert(fmt);
 
-	// TODO: what if the unpack subimage extension isn't supported?
 	push_gles2_debug(texture->renderer);
 
 	glBindTexture(GL_TEXTURE_2D, texture->tex);