op: Implement 'default_blend_color,initial_blend_color' test in blending.spec.ts (#1980)
op: Implement 'default_blend_color,initial_blend_color' test in blending.spec.ts (#1980)

This PR implements a new test to ensure the initial blending color is correctly
set at the beginning of every sub-pass.

Issue: #1835
gpuweb/cts,gpuweb/cts,gpuweb/cts