[2D_Refactor] r_clearbuffer doesn't work in TC software

[Rachael]

r_clearbuffer no longer clears the buffer in the following vid_rendermodes: 1, 2, 3

It does however work in 0 and 4.

[Graf Zahl]

From the looks of it this never got implemented in the poly renderer, so modes 2 and 3 are hardly surprising.

[Graf Zahl]

Fixed render mode 1. Yet again the base palette's unset alpha was the cause. If the software renderer didn't access the palette in seemingly a thousand places I'd just change that but knowing where that code came from I'm uncertain.