libobs-opengl: Use PBO in device_stage_texture on macOS

Previously glReadPixels was used to stage surfaces because Mac OS X at
that time supported some hardwares that didn't implement glGetTexImage.
However, OBS Studio now requires OpenGL 3.3 so that glGetTexImage on
macOS should work the same as on Linux.
The code using glReadPixels was introduced at 30af7dc. The minimum
requirements of the original OBS Studio for OSX was 10.8, which supports
hardwares with OpenGL 1.4. However, the function glGetTexImage requires
OpenGL >= 2.0.

libobs-opengl/gl-stagesurf.c

@@ -107,58 +107,6 @@ static bool can_stage(struct gs_stage_surface *dst, struct gs_texture_2d *src)
 	return true;
 }
 
-#ifdef __APPLE__
-
-/* Apparently for mac, PBOs won't do an asynchronous transfer unless you use
- * FBOs along with glReadPixels, which is really dumb. */
-void device_stage_texture(gs_device_t *device, gs_stagesurf_t *dst,
-			  gs_texture_t *src)
-{
-	struct gs_texture_2d *tex2d = (struct gs_texture_2d *)src;
-	struct fbo_info *fbo;
-	GLint last_fbo;
-	bool success = false;
-
-	if (!can_stage(dst, tex2d))
-		goto failed;
-
-	if (!gl_bind_buffer(GL_PIXEL_PACK_BUFFER, dst->pack_buffer))
-		goto failed;
-
-	fbo = get_fbo(src, dst->width, dst->height);
-
-	if (!gl_get_integer_v(GL_READ_FRAMEBUFFER_BINDING, &last_fbo))
-		goto failed_unbind_buffer;
-	if (!gl_bind_framebuffer(GL_READ_FRAMEBUFFER, fbo->fbo))
-		goto failed_unbind_buffer;
-
-	glFramebufferTexture2D(GL_READ_FRAMEBUFFER, GL_COLOR_ATTACHMENT0 + 0,
-			       src->gl_target, src->texture, 0);
-	if (!gl_success("glFrameBufferTexture2D"))
-		goto failed_unbind_all;
-
-	glReadPixels(0, 0, dst->width, dst->height, dst->gl_format,
-		     dst->gl_type, 0);
-	if (!gl_success("glReadPixels"))
-		goto failed_unbind_all;
-
-	success = true;
-
-failed_unbind_all:
-	gl_bind_framebuffer(GL_READ_FRAMEBUFFER, last_fbo);
-
-failed_unbind_buffer:
-	gl_bind_buffer(GL_PIXEL_PACK_BUFFER, 0);
-
-failed:
-	if (!success)
-		blog(LOG_ERROR, "device_stage_texture (GL) failed");
-
-	UNUSED_PARAMETER(device);
-}
-
-#else
-
 void device_stage_texture(gs_device_t *device, gs_stagesurf_t *dst,
 			  gs_texture_t *src)
 {
@@ -187,8 +135,6 @@ failed:
 	UNUSED_PARAMETER(device);
 }
 
-#endif
-
 uint32_t gs_stagesurface_get_width(const gs_stagesurf_t *stagesurf)
 {
 	return stagesurf->width;