Prevent Special:MathShowImage from showing bad content

This commit prohibits that the special-page MathShowImages
tries to display information that are not enabled via
config.

A typical example is that the Specialpage tries to
render PNG images for IE<9 even though texvc was not
set up. This was reported as

Bug: 69693
Change-Id: Ifc5888152aa02c973eab959e963085f24a7b2a83
Merge "Prevent Special:MathShowImage from showing bad content"