fix: explicitly close PdfBitmap after copy in both PDF backends

pypdfium2's to_pil() shares native buffer memory for RGBA/RGBX/L formats
via frombuffer(). The chained render().to_pil().resize() pattern allowed
the PdfBitmap to reach refcount 0 mid-expression, causing GC to invoke
FPDFBitmap_Destroy and free the native buffer while PIL still held a
dangling pointer to it — resulting in non-deterministic SIGSEGV crashes
in concurrent scenarios.

Fix: store the bitmap explicitly, copy the PIL image to detach it from
the shared native buffer, then close the bitmap under the lock before
proceeding with the resize on the independent copy.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>