svga: implement generic variable index remapping

svga: implement generic variable index remapping

The state tracker may generate shaders that use generic vs outputs /
fs inputs like:

DCL IN[0], GENERIC[0]
DCL IN[1], GENERIC[10]
DCL IN[2], GENERIC[11]

This patch remaps 0, 10, 11 to small integers like 1, 2, 3 so that we
stay inside the SVGA3D limit (8).

The remapping is done to both the vertex shader outputs and the
fragment shader inputs.  The same mapping must be used for a vs/fs
pair.

Note that 'union svga_compile_key' is now 'struct svga_compile_key'
because we needed to add the register remapping table.  The change in
size isn't really significant though (it's not a search key).

Also, add assertions when building up SVGA3D src/dst registers to we
don't try to store too large of value for the bitfield size.

Reviewed-by: José Fonseca <jfonseca@vmware.com>