Implement pixel shader derivative instructions.

deriv_rtx and dsx compile to dFdx.
deriv_rty and dxy compile to dFdy.
Check pixel shader input usage.

ps3\derivative has
dcl_texcoord v0
but v0 was being compiled as BaseColor instead of TexCoord0.
Implemented texkill
Allow texkill to operate on vec4 for SM2 onwards
Implemented pow, dp2add, lrp and texldd opcodes.
Implemented slt and sge opcodes.
Merge branch 'master' into Integer
Added HLSL_ige versions which take integer arguments.
Unsigned integer immediate values were treated as signed.