Bug 1231550 - Use DrawTarget instead of gfxContext and/or nsRenderingContext in many places in font/text code. r=jfkthame.

AutoTextRun now only needs a DrawTarget instead of an nsRenderingContext, and
similar nsRenderingContext/gfxContext-to-DrawTarget replacements can be
propagated a long way up the call graph. This patch replaces 93 occurrences of
nsRenderingContext and 135 occurrences of gfxContext with DrawTarget; that's
13% of them.

The patch is mostly plumbing changes. A couple of not-entirely-plumbing
changes:

- It adds a comment about the null check in
  gfxGlyphExtents::GetTightGlyphExtentsAppUnits().

- A couple of functions simply had an unused gfxContext or nsRenderingContext
  parameter removed, e.g. SetLineBreaks().

layout/generic/MathMLTextRunFactory.h

@@ -22,7 +22,7 @@ public:
       mSSTYScriptLevel(aSSTYScriptLevel) {}
 
   virtual void RebuildTextRun(nsTransformedTextRun* aTextRun,
-                              gfxContext* aRefContext,
+                              mozilla::gfx::DrawTarget* aRefDrawTarget,
                               gfxMissingFontRecorder* aMFR) override;
   enum {
     // Style effects which may override single character <mi> behaviour