mesa: fix sample count handling for MSRTT

this extension specifies error checking (which was not implemented)
and also sample count clamping needs to be done since the samples
specified are just min samples and not an exact param

cc: mesa-stable

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/31235>
(cherry picked from commit 894b37e060)

.pick_status.json

@@ -4294,7 +4294,7 @@
         "description": "mesa: fix sample count handling for MSRTT",
         "nominated": true,
         "nomination_type": 0,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": null,
         "notes": null

src/mesa/main/fbobject.c

@@ -3966,6 +3966,28 @@ _mesa_framebuffer_texture(struct gl_context *ctx, struct gl_framebuffer *fb,
 {
    FLUSH_VERTICES(ctx, _NEW_BUFFERS, 0);
 
+   /* EXT_multisampled_render_to_texture:
+
+      If samples is greater than the 
+      value of MAX_SAMPLES_EXT, then the error INVALID_VALUE is generated. 
+      An INVALID_OPERATION error is generated if samples is greater than
+      the maximum number of samples supported for target and its
+      internalformat. If samples is zero, then TEXTURE_SAMPLES_EXT is set
+      to zero, and FramebufferTexture2DMultisampleEXT behaves like
+      FramebufferTexture2D.
+    */
+   if (samples > ctx->Const.MaxSamples) {
+      _mesa_error(ctx, GL_INVALID_VALUE, "invalid sample count %u",
+                  samples);
+      return;
+   }
+   if (samples > ctx->Const.MaxFramebufferSamples) {
+      _mesa_error(ctx, GL_INVALID_OPERATION, "invalid sample count %u",
+                  samples);
+      return;
+   }
+
+
    simple_mtx_lock(&fb->Mutex);
    if (texObj) {
       if (attachment == GL_DEPTH_ATTACHMENT &&

src/mesa/main/renderbuffer.c

@@ -627,6 +627,27 @@ _mesa_update_renderbuffer_surface(struct gl_context *ctx,
                            last_layer);
    }
 
+   int nr_samples = rb->rtt_nr_samples;
+   if (rb->rtt_nr_samples && rb->rtt_nr_samples != resource->nr_samples) {
+      assert(!resource->nr_samples);
+      /* EXT_multisampled_render_to_texture:
+
+         Otherwise samples represents a request for a desired minimum number 
+         of samples. Since different implementations may support different 
+         sample counts for multisampled rendering, the actual number of samples 
+         allocated for the image is implementation-dependent. However, the 
+         resulting value for TEXTURE_SAMPLES_EXT is guaranteed to be greater 
+         than or equal to samples and no more than the next larger sample count 
+         supported by the implementation.
+       */
+      for (unsigned i = rb->rtt_nr_samples + 1; i <= ctx->Const.MaxFramebufferSamples; i++) {
+         if (!ctx->st->screen->is_format_supported(ctx->st->screen, format, resource->target, i, i, resource->bind))
+            continue;
+         nr_samples = i;
+         break;
+      }
+   }
+
    struct pipe_surface **psurf =
       enable_srgb ? &rb->surface_srgb : &rb->surface_linear;
    struct pipe_surface *surf = *psurf;
@@ -638,7 +659,7 @@ _mesa_update_renderbuffer_surface(struct gl_context *ctx,
        surf->texture != resource ||
        surf->width != rtt_width ||
        surf->height != rtt_height ||
-       surf->nr_samples != rb->rtt_nr_samples ||
+       surf->nr_samples != nr_samples ||
        surf->u.tex.level != level ||
        surf->u.tex.first_layer != first_layer ||
        surf->u.tex.last_layer != last_layer) {
@@ -646,7 +667,7 @@ _mesa_update_renderbuffer_surface(struct gl_context *ctx,
       struct pipe_surface surf_tmpl;
       memset(&surf_tmpl, 0, sizeof(surf_tmpl));
       surf_tmpl.format = format;
-      surf_tmpl.nr_samples = rb->rtt_nr_samples;
+      surf_tmpl.nr_samples = nr_samples;
       surf_tmpl.u.tex.level = level;
       surf_tmpl.u.tex.first_layer = first_layer;
       surf_tmpl.u.tex.last_layer = last_layer;