renderer: allow blending final output

This is pretty trivial to support, and moves things further in
consideration of haasn#118.

meson.build

@@ -2,7 +2,7 @@ project('libplacebo', ['c', 'cpp'],
   license: 'LGPL2.1+',
   default_options: ['c_std=c99', 'cpp_std=c++11', 'warning_level=2'],
   meson_version: '>=0.51',
-  version: '2.103.0',
+  version: '2.104.0',
 )
 
 # Version number

src/include/libplacebo/renderer.h

@@ -120,6 +120,11 @@ struct pl_render_params {
     // If NULL, this feature is disabled.
     const struct pl_cone_params *cone_params;
 
+    // Configures output blending. When rendering to the final target, the
+    // framebuffer contents will be blended using this blend mode. Requires
+    // that the target format has PL_FMT_CAP_BLENDABLE. NULL disables blending.
+    const struct pl_blend_params *blend_params;
+
     // List of custom user shaders / hooks.
     // See <libplacebo/shaders/custom.h> for more information.
     const struct pl_hook * const *hooks;

src/renderer.c

@@ -1572,6 +1572,7 @@ static bool pass_output_target(struct pl_renderer *rr, struct pass_state *pass,
         bool ok = pl_dispatch_finish(rr->dp, &(struct pl_dispatch_params) {
             .shader = &sh,
             .target = plane->texture,
+            .blend_params = params->blend_params,
             .rect = {
                 .x0 = flipped_x ? rx1 : rx0,
                 .y0 = flipped_y ? ry1 : ry0,