[Core Graphics] Disambiguate the CGContext PDF endPage (#3139)

This changes how we import CGPDFContextEndPage to be
CGContext.endPDFPage(), instead of .endPage() which conflicts with
another CGContext function. Also does .beginPDFPage() for consistency,
which is consistent with e.g. drawPDFPage().

Author: milseman

apinotes/CoreGraphics.apinotes

@@ -386,9 +386,9 @@ Functions:
 - Name: CGPDFContextClose
   SwiftName: CGContext.closePDF(self:)
 - Name: CGPDFContextBeginPage
-  SwiftName: CGContext.beginPage(self:_:)
+  SwiftName: CGContext.beginPDFPage(self:_:)
 - Name: CGPDFContextEndPage
-  SwiftName: CGContext.endPage(self:)
+  SwiftName: CGContext.endPDFPage(self:)
 - Name: CGPDFContextAddDocumentMetadata
   SwiftName: CGContext.addDocumentMetadata(self:_:)
 - Name: CGPDFContextSetURLForRect