gallium: fix SSE shadow texture instructions

When sampling a 2D shadow map we need 3 texcoord components, not 2.
The third component (distance from light source) is compared against
the texture sample to return the result (visible vs. occluded).

Also, enable proper handling of TGSI_TEXTURE_SHADOW targets in Mesa->TGSI
translation.  There's a possibility for breakage in gallium drivers if
they fail to handle the TGSI_TEXTURE_SHADOW1D / TGSI_TEXTURE_SHADOW2D /
TGSI_TEXTURE_SHADOWRECT texture targets for TGSI_OPCODE_TEX/TXP instructions,
but that should be easy to fix.

With these changes, progs/demos/shadowtex.c renders properly again with
softpipe.

src/gallium/auxiliary/tgsi/tgsi_sse2.c

@@ -1467,15 +1467,15 @@ emit_tex( struct x86_function *func,
 
    switch (inst->InstructionExtTexture.Texture) {
    case TGSI_TEXTURE_1D:
-   case TGSI_TEXTURE_SHADOW1D:
       count = 1;
       break;
    case TGSI_TEXTURE_2D:
    case TGSI_TEXTURE_RECT:
-   case TGSI_TEXTURE_SHADOW2D:
-   case TGSI_TEXTURE_SHADOWRECT:
       count = 2;
       break;
+   case TGSI_TEXTURE_SHADOW1D:
+   case TGSI_TEXTURE_SHADOW2D:
+   case TGSI_TEXTURE_SHADOWRECT:
    case TGSI_TEXTURE_3D:
    case TGSI_TEXTURE_CUBE:
       count = 3;

src/mesa/state_tracker/st_mesa_to_tgsi.c

@@ -158,12 +158,6 @@ map_texture_target(
     GLuint textarget,
     GLboolean shadow )
 {
-#if 1
-   /* XXX remove this line after we've checked that the rest of gallium
-    * can handle the TGSI_TEXTURE_SHADOWx tokens.
-    */
-   shadow = GL_FALSE;
-#endif
    switch( textarget ) {
    case TEXTURE_1D_INDEX:
       if (shadow)