graft: fix invisible text appearing after strip_invisible_text

strip_invisible_text resets the text render mode on each `BT` (begin text) command. However the text state is not actually reset for each text element, only for each page.

The pdf reference says:

> The text state operators can appear outside text objects, and the values they set
> are retained across text objects in a single content stream. Like other graphics
> state parameters, these parameters are initialized to their default values at the
> beginning of each page.
>
> -- https://opensource.adobe.com/dc-acrobat-sdk-docs/pdfstandards/pdfreference1.7old.pdf#page=397

With the current implementation, a text object is only deleted if it contains a `3 Tr` command (setting the text rendering mode to invalid). However the rendering mode may be set once and then not changed for multiple text objects or set outside of a text object.
In that case only the first text object (which contains the `3 Tr`-command) is removed. This not only leaves the other text objects in the pdf, but also makes them visible, since the text object that contained the `3 Tr`-command is removed.

This PR updates `strip_invisible_text` to not reset the rendering mode for each object and to keep track of the rendering mode when the graphic state is pushed/popped.

src/ocrmypdf/_graft.py

@@ -57,27 +57,34 @@ def _update_resources(
         fonts[font_key] = font
 
 
+
 def strip_invisible_text(pdf: Pdf, page: Page):
     stream = []
     in_text_obj = False
-    render_mode = 0
+    render_mode_stack = [0]
     text_objects = []
 
     for operands, operator in parse_content_stream(page, ''):
+        if operator == Operator('Tr'):
+            render_mode_stack[-1] = operands[0]
+
+        if operator == Operator('q'):
+            render_mode_stack.append(render_mode_stack[-1])
+
+        if operator == Operator('Q'):
+            render_mode_stack.pop()
+
         if not in_text_obj:
             if operator == Operator('BT'):
                 in_text_obj = True
-                render_mode = 0
                 text_objects.append((operands, operator))
             else:
                 stream.append((operands, operator))
         else:
-            if operator == Operator('Tr'):
-                render_mode = operands[0]
             text_objects.append((operands, operator))
             if operator == Operator('ET'):
                 in_text_obj = False
-                if render_mode != 3:
+                if render_mode_stack[-1] != 3:
                     stream.extend(text_objects)
                 text_objects.clear()