fix: consider dim introduced transparency for blur

[rti]

The blur rendering logic includes the optimization to render blur only for transparent surfaces. This patch considers surfaces as transparent also when the transparency is only introduced by the dim color.

We can now have windows, that are opaque by default become transparent with blurred background as soon as they loose focus.

Example configuration:

blur on
blur_passes 3
blur_radius 5

default_dim_inactive 0.15
dim_inactive_colors.unfocused #00000000

[WillPower3309]

Good catch!