vo_gpu: add RA_CAP for gl_NumWorkGroups

SPIRV-Cross doesn't support this for the time being. It's possible this
could go away again at a later date.

video/out/gpu/ra.h

@@ -54,6 +54,7 @@ enum {
     RA_CAP_GATHER         = 1 << 9, // supports textureGather in GLSL
     RA_CAP_FRAGCOORD      = 1 << 10, // supports reading from gl_FragCoord
     RA_CAP_PARALLEL_COMPUTE  = 1 << 11, // supports parallel compute shaders
+    RA_CAP_NUM_GROUPS     = 1 << 12, // supports gl_NumWorkGroups
 };
 
 enum ra_ctype {

video/out/opengl/ra_gl.c

@@ -101,6 +101,7 @@ static int ra_init_gl(struct ra *ra, GL *gl)
         {RA_CAP_TEX_1D,             MPGL_CAP_1D_TEX},
         {RA_CAP_TEX_3D,             MPGL_CAP_3D_TEX},
         {RA_CAP_COMPUTE,            MPGL_CAP_COMPUTE_SHADER},
+        {RA_CAP_NUM_GROUPS,         MPGL_CAP_COMPUTE_SHADER},
         {RA_CAP_NESTED_ARRAY,       MPGL_CAP_NESTED_ARRAY},
     };
 

video/out/vulkan/ra_vk.c

@@ -209,7 +209,7 @@ struct ra *ra_create_vk(struct mpvk_ctx *vk, struct mp_log *log)
     ra->max_pushc_size = vk->limits.maxPushConstantsSize;
 
     if (vk->pool_compute) {
-        ra->caps |= RA_CAP_COMPUTE;
+        ra->caps |= RA_CAP_COMPUTE | RA_CAP_NUM_GROUPS;
         // If we have more compute queues than graphics queues, we probably
         // want to be using them. (This seems mostly relevant for AMD)
         if (vk->pool_compute->num_queues > vk->pool_graphics->num_queues)