Fix loading legacy brush masks

If there are two resources in two storages that have the same
md5sum (square_rough.png and texture_3.png), we need to make
sure that the one with the filename stored in the preset can
be found, so now resources(md5, filename, name) returns all
matching resources.

BUG:441350
BUG:441347
(cherry picked from commit 91bbbd5f)