Change alpha channel to non-transparent

[breml]

trafficstars

With the change introduced in https://github.com/tinygo-org/drivers/pull/472 the alpha channel of the color used with the microbitmatrix is no longer ignored. For a pixel to light in full brightness, the alpha channel needs to be set to 0 (no transparency = full brightness).

[breml]

While https://github.com/tinygo-org/drivers/pull/472 is merged to the dev branch, this feature has not yet landed in the release branch and therefore we might want to wait with the merge of this PR until said feature is finally released.