All bound layers (from first_layer to last_layer) should be cleared.
This uses a vertex shader which outputs gl_Layer = gl_InstanceID, so each
instance goes to a different layer. By rendering a quad and setting
the instance count to the number of layers, it will trivially clear all
layers.
This requires AMD_vertex_shader_layer (or PIPE_CAP_TGSI_VS_LAYER), which only
radeonsi supports at the moment. r600 could do this too. Standard DX11
hardware will have to use a geometry shader though, which has higher overhead.
There are also some u_simple_shaders changes.
On r300, the TGSI_SEMANTIC_COLOR varying is a fixed-point number clamped
to the range [0,1] and limited to 12 bits of precision. Therefore we can't
use it for passing through a clear color in order to clear high precision
texture formats.
This also makes u_blitter use only one vertex shader instead of two.
New shaders:
* Fragment shader which writes depth sampled from a texture
* Fragment shader which copies COLOR[0] to multiple render targets
Additional improvements:
* The fragment 'tex' shaders now take a sampler type (TGSI_TEXTURE_*)
so that they can sample from any type of texture, not only from a 2D one.
Any driver who needs a copy of the shader tokens must organize to
do so itself. This has been the case for a long time, but there
was still defensive code in the state tracker, which is now removed.
Any bugs resulting from this need to be fixed in the offending driver...
