[Mesa3d-dev] [rfc] to SpanRenderStart to avoid texture mapping overheads

[Dave A. <ai...@gm...>]

So in classic drivers we can hit swrast fallbacks for things like
ReadPixels where we know we aren't going to have to want to touch
textures at all. This would be useful for the r300 driver where Maciej
is working on texture tiling will allow us to avoid the untiling
overheads that mapping textures requires for most sw fallbacks.

There may be other operations where we can do this also. For render to
texture scenarios the driver should still map the textures via the
FBOs anyways.

Dave.