[Impeller] Reset transient HostBuffer on raster snapshots to prevent memory leak - #192937

Fixes #192097

When rasterizing offscreen snapshots through RenderRepaintBoundary.toImage(), SnapshotControllerImpeller::DoMakeRasterSnapshot passed /*reset_host_buffer=*/false to impeller::DisplayListToTexture. Because the transient data and index host buffers were never reset upon snapshot completion, each snapshot accumulated vertex and uniform data in HostBuffer which was never cleaned up.

Looking at the history of this file, I don't see anything which suggests a reason why this may have been intentionally set to false, so this PR sets it to true.

I also added a unit test SnapshotControllerImpeller.MakeImpellerSnapshotSyncRecyclesHostBuffer in gpu_surface_metal_impeller_unittests.mm that executes repeated snapshots against a mock delegate and asserts that the total allocated buffer count remains bounded at 1.

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