render/gles2: add wlr_gles2_renderer_get_buffer_fbo()

Replacement for wlr_gles2_renderer_get_current_fbo(). Wayfire uses
it for instance.

include/wlr/render/gles2.h

@@ -10,6 +10,7 @@
 #define WLR_RENDER_GLES2_H
 
 #include <GLES2/gl2.h>
+
 #include <wlr/render/wlr_renderer.h>
 
 struct wlr_egl;
@@ -31,8 +32,8 @@ struct wlr_renderer *wlr_gles2_renderer_create_with_drm_fd(int drm_fd);
 struct wlr_renderer *wlr_gles2_renderer_create(struct wlr_egl *egl);
 
 struct wlr_egl *wlr_gles2_renderer_get_egl(struct wlr_renderer *renderer);
-bool wlr_gles2_renderer_check_ext(struct wlr_renderer *renderer,
-	const char *ext);
+bool wlr_gles2_renderer_check_ext(struct wlr_renderer *renderer, const char *ext);
+GLuint wlr_gles2_renderer_get_buffer_fbo(struct wlr_renderer *renderer, struct wlr_buffer *buffer);
 
 struct wlr_gles2_texture_attribs {
 	GLenum target; /* either GL_TEXTURE_2D or GL_TEXTURE_EXTERNAL_OES */

render/gles2/renderer.c

@@ -271,6 +271,26 @@ static struct wlr_render_pass *gles2_begin_buffer_pass(struct wlr_renderer *wlr_
 	return &pass->base;
 }
 
+GLuint wlr_gles2_renderer_get_buffer_fbo(struct wlr_renderer *wlr_renderer,
+		struct wlr_buffer *wlr_buffer) {
+	struct wlr_gles2_renderer *renderer = gles2_get_renderer(wlr_renderer);
+	GLuint fbo = 0;
+
+	struct wlr_egl_context prev_ctx = {0};
+	wlr_egl_save_context(&prev_ctx);
+	if (!wlr_egl_make_current(renderer->egl)) {
+		return 0;
+	}
+
+	struct wlr_gles2_buffer *buffer = gles2_buffer_get_or_create(renderer, wlr_buffer);
+	if (buffer) {
+		fbo = gles2_buffer_get_fbo(buffer);
+	}
+
+	wlr_egl_restore_context(&prev_ctx);
+	return fbo;
+}
+
 static struct wlr_render_timer *gles2_render_timer_create(struct wlr_renderer *wlr_renderer) {
 	struct wlr_gles2_renderer *renderer = gles2_get_renderer(wlr_renderer);
 	if (!renderer->exts.EXT_disjoint_timer_query) {