[TASKMGR] misbehaves upon drawing when maximizing and restoring in the performance tab

This is verified to be a bug in our taskmgr itself, because it can be observed if we do run our taskmgr binary on XPSP3.

• Reproduction steps
• open the performance tab and watch its graphs for a moment
• maximize the window by double-clicking the titlebar
• restore the window by double-clicking the titlebar again

Observed result
painting_glitch.PNG
I tested the taskmgr binary of 0.4.15-dev-6712-g46b4b55 here
All taskmgr binaries of all release branches down to at least 0.4.7-release-266-gf643e3b are affected like that. It is nothing that would have been recently introduced.