use large/medium/similar to define scene change result
fix the condition of scene change flag and comments
Add encoder tests with deblocking and/or denoising

This increases the coverage of assembly routines (and encoder
features in general) tested by the codec unit test.
Use the SIGN_EXTENSION macro where possible

This shortens the x86 assembly by 134 lines in total.
Merge pull request #496 from mstorsjo/use-sign-extend-macro

Use the SIGN_EXTENSION macro where possible
Merge pull request #483 from volvet/develop_b

use large/medium/similar to define scene change result
Merge pull request #503 from mstorsjo/encoder-test-coverage

Add encoder tests with deblocking and/or denoising