Wrap AGG code in namespace pdfium.

This helps make PDFium's copy of AGG unique, so embedders can avoid
conflicts when linking PDFium together with another library that comes
with a different copy of AGG.

To make this compile, wrap CFX_AggDeviceDriver in namespace pdfium as
well. Also fix some nits in fx_agg_driver.cpp along the way.

Bug: pdfium:1595
Change-Id: I6c34f28e6c07d09aef55722c430e0647826fd81d
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/74374
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/third_party/agg23/agg_scanline_u.h b/third_party/agg23/agg_scanline_u.h
index 844dc9a..1db31c3 100644
--- a/third_party/agg23/agg_scanline_u.h
+++ b/third_party/agg23/agg_scanline_u.h
@@ -24,6 +24,8 @@
 #ifndef AGG_SCANLINE_U_INCLUDED
 #define AGG_SCANLINE_U_INCLUDED
 #include "agg_array.h"
+namespace pdfium
+{
 namespace agg
 {
 template<class CoverT> class scanline_u 
@@ -147,4 +149,5 @@
 };
 typedef scanline_u<int8u> scanline_u8;
 }
+}  // namespace pdfium
 #endif