We'd first set the brush, then change tools, which would again set the
brush to what it was before (switching tools tries to remember its last
used brush). Instead, switch the brush after switching the tool.
Also added a brush API check to test if the brush is valid for the
current mode, so that we don't end up switching tools in that case. This
matches previous behavior where this would be checked as part of setting
the brush already, causing the function to early exit before the tool is
changed.
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