Fix --enable-webgl-software-rendering with drivers that don't support OpenGL

--enable-webgl-software-rendering is supposed to be used together
with mesa llvmpipe / opengl32sw.dll. However, Chromium did still use
the built-in GPU blacklist to disable features based on the hardware
GPU driver.

Avoid this by always passing --ignore-gpu-blacklist for this mode.

Task-number: QTBUG-69236
Change-Id: I430d101f6eac64478585de54f705e76859c80597
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>