Auto merge of #7912 - eefriedman:compositor-layer, r=mrobinson
Auto merge of #7912 - eefriedman:compositor-layer, r=mrobinson

Make sure to correctly destroy old layers in collect_old_layers_for_pipeline.

The previous code would skip calling clear_all_tiles(), which led to a
panic in rust-layers. ("You should have disposed of the pixmap properly
with destroy()! This pixmap will leak!")

Ran into this messing around with pdf.js; no minimized testcase.
Maybe related to #7895.

r? @pcwalton

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