* scm/output-tex.scm (font-command): use ly: functions to

* scm/output-tex.scm (font-command): use ly: functions to
construct name.
(define-fonts): don't use alist, just dump \font definitions.
(font-load-command): calculate proper scaling by taking
outputscale from paper argument.

* lily/stencil-scheme.cc (fontify_atom): insert Font_metric into
expression.

* lily/paper-outputter.cc (output_header): pass paper arg to
define-fonts

* lily/paper-def.cc (font_descriptions): dump list of
Scaled_font_metrics
(find_scaled_font): take Font_metric argument, store
Scaled_font_metrics in hash-table containing (size ->
Scaled_font_metrics) alist.
(get_dimension): rename from get_realvar

* lily/font-metric.cc (LY_DEFINE): ly:font-design-size,
ly:font-magnification, ly:font-name: new functions