fixed auto viewport without attachements

src/glow/objects/Framebuffer.cc

@@ -247,7 +247,8 @@ bool BoundFramebuffer::isCurrent() const
 
 void BoundFramebuffer::updateViewport()
 {
-    tg::isize3 dim(32 * 1024);
+    auto sentinel = tg::max<int>();
+    auto dim = tg::isize3(sentinel);
     if (buffer->mDepthAttachment.texture)
         dim = tg::min(dim, buffer->mDepthAttachment.texture->getDimensions());
     if (buffer->mStencilAttachment.texture)
@@ -255,6 +256,7 @@ void BoundFramebuffer::updateViewport()
     for (auto const& a : buffer->mColorAttachments)
         if (a.texture)
             dim = tg::min(dim, a.texture->getDimensions());
+    if (dim != tg::isize3(sentinel)) // at least one texture
         glViewport(0, 0, dim.width, dim.height);
 }