Cleanup CPDF_RenderOptions colour members.

This CL removes the fore/back colours in favour of just using the
colours inline and converts the m_ColorMode to an enum.

Change-Id: I1796e200d5c470680126af8625d45fc3062a1042
Reviewed-on: https://pdfium-review.googlesource.com/7751
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Reviewed-by: Nicolás Peña <npm@chromium.org>
Commit-Queue: dsinclair <dsinclair@chromium.org>
diff --git a/fpdfsdk/fpdfformfill.cpp b/fpdfsdk/fpdfformfill.cpp
index f2ae621..24cd64e 100644
--- a/fpdfsdk/fpdfformfill.cpp
+++ b/fpdfsdk/fpdfformfill.cpp
@@ -126,11 +126,9 @@
       options.m_Flags &= ~RENDER_CLEARTYPE;
 
     // Grayscale output
-    if (flags & FPDF_GRAYSCALE) {
-      options.m_ColorMode = RENDER_COLOR_GRAY;
-      options.m_ForeColor = 0;
-      options.m_BackColor = 0xffffff;
-    }
+    if (flags & FPDF_GRAYSCALE)
+      options.m_ColorMode = CPDF_RenderOptions::kGray;
+
     options.m_bDrawAnnots = flags & FPDF_ANNOT;
 
 #ifdef PDF_ENABLE_XFA