Support for TeX fonts: font search


The TeX font mapper font driver:
A "delegate" font driver.
When a font is requested to open, try to open it by calling other font drivers.

Example: 
a font cmr10.pk at 300 DPI is requested to open

Search cmr10.300pk (PK format). If found, use it.
Search cmr10.300gf (GF format). If found, use it.
Search cmr10.pfm (Type 1 format). If found, use it.
Search cmr10.vf (VF format). If found, use it.
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