Auto merge of #201 - pcwalton:fallible-create-source-surface-from-data, r=mbrubeck
Auto merge of #201 - pcwalton:fallible-create-source-surface-from-data, r=mbrubeck

Make `create_source_surface_from_data` fallible.

Some Azure backends will return a null source surface if the size is
zero, which was causing a crash. This patch forces callers to handle
this case by returning an option.

r? @mbrubeck

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mbrubeck/rust-azure,mbrubeck/rust-azure,mbrubeck/rust-azure,vvuk/rust-azure,dzbarsky/rust-azure,vvuk/rust-azure,servo/rust-azure,dzbarsky/rust-azure,servo/rust-azure,servo/rust-azure,mbrubeck/rust-azure,dzbarsky/rust-azure,vvuk/rust-azure,dzbarsky/rust-azure,servo/rust-azure,dzbarsky/rust-azure,vvuk/rust-azure,vvuk/rust-azure,mbrubeck/rust-azure,servo/rust-azure