Fix similar to the fix in https://developer.blender.org/D14322
Corrects the calculation of the Frame Node label size, making it independent of the line-width user setting.
U.dpi is actually DPI divided by U.pixelsize, and U.pixelsize is calculated from line-width. Multiplying by U.pixelsize breaks the connection between line-width and labelsize, labelsize stays the same now, regardless of the line-width setting.