Rename GLES2 driver to OpenGL to prepare for the upgrade to GLES3

- Use lowercase driver names for the `--rendering-driver`
  command line argument.

drivers/opengl/rasterizer_opengl.h

@@ -0,0 +1,92 @@
+/*************************************************************************/
+/*  rasterizer_opengl.h                                                  */
+/*************************************************************************/
+/*                       This file is part of:                           */
+/*                           GODOT ENGINE                                */
+/*                      https://godotengine.org                          */
+/*************************************************************************/
+/* Copyright (c) 2007-2021 Juan Linietsky, Ariel Manzur.                 */
+/* Copyright (c) 2014-2021 Godot Engine contributors (cf. AUTHORS.md).   */
+/*                                                                       */
+/* Permission is hereby granted, free of charge, to any person obtaining */
+/* a copy of this software and associated documentation files (the       */
+/* "Software"), to deal in the Software without restriction, including   */
+/* without limitation the rights to use, copy, modify, merge, publish,   */
+/* distribute, sublicense, and/or sell copies of the Software, and to    */
+/* permit persons to whom the Software is furnished to do so, subject to */
+/* the following conditions:                                             */
+/*                                                                       */
+/* The above copyright notice and this permission notice shall be        */
+/* included in all copies or substantial portions of the Software.       */
+/*                                                                       */
+/* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,       */
+/* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF    */
+/* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.*/
+/* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY  */
+/* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,  */
+/* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE     */
+/* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.                */
+/*************************************************************************/
+
+#ifndef RASTERIZER_OPENGL_H
+#define RASTERIZER_OPENGL_H
+
+#include "drivers/opengl/rasterizer_platforms.h"
+#ifdef OPENGL_BACKEND_ENABLED
+
+#include "rasterizer_canvas_opengl.h"
+#include "rasterizer_scene_opengl.h"
+#include "rasterizer_storage_opengl.h"
+#include "servers/rendering/renderer_compositor.h"
+
+class RasterizerOpenGL : public RendererCompositor {
+private:
+	uint64_t frame = 1;
+	float delta = 0;
+
+	double time_total = 0.0;
+	double time_scale = 1.0;
+
+protected:
+	RasterizerCanvasOpenGL canvas;
+	RasterizerStorageOpenGL storage;
+	RasterizerSceneOpenGL scene;
+
+	void _blit_render_target_to_screen(RID p_render_target, const Rect2 &p_screen_rect);
+
+public:
+	RendererStorage *get_storage() { return &storage; }
+	RendererCanvasRender *get_canvas() { return &canvas; }
+	RendererSceneRender *get_scene() { return &scene; }
+
+	void set_boot_image(const Ref<Image> &p_image, const Color &p_color, bool p_scale, bool p_use_filter = true);
+
+	void initialize();
+	void begin_frame(double frame_step);
+
+	void prepare_for_blitting_render_targets();
+	void blit_render_targets_to_screen(DisplayServer::WindowID p_screen, const BlitToScreen *p_render_targets, int p_amount);
+
+	void end_frame(bool p_swap_buffers);
+
+	void finalize() {}
+
+	static RendererCompositor *_create_current() {
+		return memnew(RasterizerOpenGL);
+	}
+
+	static void make_current() {
+		_create_func = _create_current;
+	}
+
+	virtual bool is_low_end() const { return true; }
+	uint64_t get_frame_number() const { return frame; }
+	double get_frame_delta_time() const { return delta; }
+
+	RasterizerOpenGL();
+	~RasterizerOpenGL() {}
+};
+
+#endif // OPENGL_BACKEND_ENABLED
+
+#endif