GrContext.createBackendTexture() crash

[kyamagu]

Describe the bug
After upgrading skia milestone to 84, GrContext.createBackendTexture() crashes for the signatures taking SurfaceCharacterization object.

To Reproduce
Call GrContext.createBackendTexture() with SurfaceCharacterization.

Expected behavior
Should not crash.

Desktop (please complete the following information):

• OS: macOS 10.15.4
• Architecture x86_64
• Python 3.7
• skia-python version 0.0.4

Additional context
N/A