Initial support for floating point and signed texture formats.

New "FetchTexelFuncF()" function returns texels in floating point format.
Only used for depth component images at this time.
Changed original FetchTexelFunc to return GLchan instead of GLvoid, removing
need for a bunch of ugly casts.

src/mesa/swrast/s_nvfragprog.c

@@ -56,6 +56,7 @@ fetch_texel( GLcontext *ctx, const GLfloat texcoord[4], GLfloat lambda,
    GLchan rgba[4];
    SWcontext *swrast = SWRAST_CONTEXT(ctx);
 
+   /* XXX use a float-valued TextureSample routine here!!! */
    swrast->TextureSample[unit](ctx, unit, ctx->Texture.Unit[unit]._Current,
                                1, (const GLfloat (*)[4]) texcoord,
                                &lambda, &rgba);