Change hookspec to migration parameters for generate_pdf to options object

Breaking change for PDF rendering plugins (although none are known to exist).
This provides better separation of Ghostscript specific concerns from
the generic plugin interface.

tests/plugins/gs_render_soft_error.py

@@ -23,7 +23,7 @@ def generate_pdfa(
     pdf_pages,
     pdfmark,
     output_file,
-    compression,
+    context,
     pdf_version,
     pdfa_part,
     stop_on_soft_error,
@@ -34,8 +34,7 @@ def generate_pdfa(
             pdf_pages=pdf_pages,
             pdfmark=pdfmark,
             output_file=output_file,
-            compression=compression,
-            color_conversion_strategy='LeaveColorUnchanged',
+            context=context,
             pdf_version=pdf_version,
             pdfa_part=pdfa_part,
             progressbar_class=None,