Changes seamful cubemap emulation to always compute the derivative,
emulating the bias parameter by scaling the provided derivatives.
This results in more accurate mipmap levels for seams within primitives.
There are some artifacts as a result of how derivatives are calculated,
but this matches the native driver.

Bug: angleproject:3243
Bug: angleproject:3732
Change-Id: Icb976e2a7e14cb4210645571edc037d4e607bd0d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1754383
Commit-Queue: James Dong <dongja@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>