mesa: Ignore the depth buffer when computing framebuffer floatness

Not that Z32F is especially common, but we shouldn't consider it to
imply that the color buffers are also float, which is what floatMode
is meant to mean.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/10491>

src/mesa/main/framebuffer.c

@@ -447,6 +447,8 @@ _mesa_update_framebuffer_visual(struct gl_context *ctx,
 
    fb->Visual.floatMode = GL_FALSE;
    for (unsigned i = 0; i < BUFFER_COUNT; i++) {
+      if (i == BUFFER_DEPTH)
+         continue;
       if (fb->Attachment[i].Renderbuffer) {
          const struct gl_renderbuffer *rb = fb->Attachment[i].Renderbuffer;
          const mesa_format fmt = rb->Format;