renderer: fix ssbo != -1 comparison.

Use the GL define instead.

src/vrend_renderer.c

@@ -3226,7 +3226,7 @@ static void vrend_draw_bind_ssbo_shader(struct vrend_context *ctx, int shader_ty
       res = (struct vrend_resource *)ssbo->res;
       glBindBufferRange(GL_SHADER_STORAGE_BUFFER, i, res->id,
                         ssbo->buffer_offset, ssbo->buffer_size);
-      if (ctx->sub->prog->ssbo_locs[shader_type][i] != -1)
+      if (ctx->sub->prog->ssbo_locs[shader_type][i] != GL_INVALID_INDEX)
          glShaderStorageBlockBinding(ctx->sub->prog->id, ctx->sub->prog->ssbo_locs[shader_type][i], i);
    }
 }