LibPDF: Mark text rendering matrix dirty after changing it in text_begin

A certain PDF was drawing some text used `9 0 0 9 474.54 700.6801 Tm`
to set the text matrix to a matrix that scaled by 9 in one text object.

Then, after ending that text object, it had the following new text
object which contained nothing that invalidated the text matrix:

```
BT
/F1 7 Tf
/DeviceRGB CS
0 0 0 SC
10 TL
86.37849 21.908 Td
(Authorized licensed use limited to: ...) Tj
ET
```

`BT` did reset it as required, but since we didn't mark the matrix
as dirty, we never recomputed it and drew the additional text scaled
up 9x.

Userland/Libraries/LibPDF/Renderer.cpp

@@ -416,6 +416,7 @@ RENDERER_HANDLER(text_begin)
 {
     m_text_matrix = Gfx::AffineTransform();
     m_text_line_matrix = Gfx::AffineTransform();
+    m_text_rendering_matrix_is_dirty = true;
     return {};
 }