Triple default $wgMaxShellMemory (raise to 300 MB)

100 MB is no longer sufficient.

That value was chosen back in 2006 (r14795); the world has largely
moved on to 64-bit programs since then, and applications such as
ImageMagick seem to use 16-bit precision for each image channel.
It's not enough to scale regular large-ish images on default system
settings anymore.

Bug: 34241
Change-Id: I66bbe12925689f9273f3b5fb29adc8c15c828c70
(cherry picked from commit 45d3656a4c8e169cadc5bda339296cf0aa00556f)