vo_opengl: support user compute shaders

These are identical to regular fragment shader hooks, but with extra
metadata indicating the preferred block size.

DOCS/man/options.rst

@@ -4271,6 +4271,17 @@ The following video options are currently all specific to ``--vo=opengl`` and
         should be stored in the texture, up to 4 (rgba). By default, this value
         is equal to the number of components in HOOKED.
 
+    COMPUTE bw bh
+        Specifies that this shader should be treated as a compute shader, with
+        the block size bw and bh. The compute shader will be dispatched with
+        however many blocks are necessary to completely tile over the output.
+        Compute shaders in mpv are treated similarly to fragment shaders, and
+        are still required to produce an output color. In addition, mpv
+        provides a special function NAME_map(id) to map from the global ID
+        space to the texture coordinates for all bound textures. The only real
+        difference is the fact that you can use shared memory inside compute
+        shaders.
+
     Each bound texture (via ``BIND``) will make available the following
     definitions to that shader pass, where NAME is the name of the bound
     texture:

video/out/opengl/user_shaders.c

@@ -269,6 +269,14 @@ bool parse_user_shader_pass(struct mp_log *log, struct bstr *body,
             continue;
         }
 
+        if (bstr_eatstart0(&line, "COMPUTE")) {
+            if (bstr_sscanf(line, "%d %d", &out->compute_w, &out->compute_h) != 2) {
+                mp_err(log, "Error while parsing COMPUTE!\n");
+                return false;
+            }
+            continue;
+        }
+
         // Unknown command type
         mp_err(log, "Unrecognized command '%.*s'!\n", BSTR_P(line));
         return false;

video/out/opengl/user_shaders.h

@@ -66,6 +66,8 @@ struct gl_user_shader {
     struct szexp height[MAX_SZEXP_SIZE];
     struct szexp cond[MAX_SZEXP_SIZE];
     int components;
+    int compute_w;
+    int compute_h;
 };
 
 // Parse the next shader pass from 'body'. Returns false if the end of the

video/out/opengl/video.c

@@ -1339,6 +1339,7 @@ static void hook_prelude(struct gl_video *p, const char *name, int id,
     GLSLHF("#define %s_size texture_size%d\n", name, id);
     GLSLHF("#define %s_rot texture_rot%d\n", name, id);
     GLSLHF("#define %s_pt pixel_size%d\n", name, id);
+    GLSLHF("#define %s_map texmap%d\n", name, id);
     GLSLHF("#define %s_mul %f\n", name, tex.multiplier);
 
     // Set up the sampling functions
@@ -1903,6 +1904,7 @@ static void user_hook(struct gl_video *p, struct img_tex tex,
     pass_describe(p, "user shader: %.*s (%s)", BSTR_P(shader->desc),
                   plane_names[tex.type]);
 
+    compute_size_minimum(p, shader->compute_w, shader->compute_h);
     load_shader(p, shader->pass_body);
     GLSLF("color = hook();\n");