render: add get native paint target of renderer

Add wlr_pixman_buffer_get_current_image for wlr_pixman_renderer.
Add wlr_gles2_buffer_get_current_fbo for wlr_gles2_renderer.

Allow get the FBO/pixman_image_t, the compositor can be add some
action for FBO(for eg, attach a depth buffer), or without pixman
render to pixman_image_t(for eg, use QPainter of Qt instead of pixman).

include/wlr/render/gles2.h

@@ -21,6 +21,10 @@ 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);
+/**
+ * Returns the OpenGL FBO of current buffer.
+ */
+GLuint wlr_gles2_renderer_get_current_fbo(struct wlr_renderer *wlr_renderer);
 
 struct wlr_gles2_texture_attribs {
 	GLenum target; /* either GL_TEXTURE_2D or GL_TEXTURE_EXTERNAL_OES */

include/wlr/render/pixman.h

@@ -14,6 +14,11 @@
 #include <wlr/render/wlr_renderer.h>
 
 struct wlr_renderer *wlr_pixman_renderer_create(void);
+/**
+ * Returns the image of current buffer.
+ */
+pixman_image_t *wlr_pixman_renderer_get_current_image(
+	struct wlr_renderer *wlr_renderer);
 
 bool wlr_renderer_is_pixman(struct wlr_renderer *wlr_renderer);
 bool wlr_texture_is_pixman(struct wlr_texture *texture);

render/gles2/renderer.c

@@ -919,3 +919,9 @@ bool wlr_gles2_renderer_check_ext(struct wlr_renderer *wlr_renderer,
 	struct wlr_gles2_renderer *renderer = gles2_get_renderer(wlr_renderer);
 	return check_gl_ext(renderer->exts_str, ext);
 }
+
+GLuint wlr_gles2_renderer_get_current_fbo(struct wlr_renderer *wlr_renderer) {
+	struct wlr_gles2_renderer *renderer = gles2_get_renderer(wlr_renderer);
+	assert(renderer->current_buffer);
+	return renderer->current_buffer->fbo;
+}

render/pixman/renderer.c

@@ -567,3 +567,10 @@ pixman_image_t *wlr_pixman_texture_get_image(struct wlr_texture *wlr_texture) {
 	struct wlr_pixman_texture *texture = get_texture(wlr_texture);
 	return texture->image;
 }
+
+pixman_image_t *wlr_pixman_renderer_get_current_image(
+		struct wlr_renderer *wlr_renderer) {
+	struct wlr_pixman_renderer *renderer = get_renderer(wlr_renderer);
+	assert(renderer->current_buffer);
+	return renderer->current_buffer->image;
+}