i915: Add optional support for ARB_fragment_shader under a driconf option.

Other vendors have enabled ARB_fragment_shader as part of OpenGL 2.0
enablement even on hardware like the 915 with no dynamic branching or
dFdx/dFdy support.  But for now we'll leave it disabled because we don't
do any flattening of ifs or loops, which is rather restrictive.

This support is not complete, and may be unstable depending on your shaders.
It passes 10/15 of the piglit glsl tests, but hangs on glean glsl1.

src/mesa/drivers/dri/intel/intel_screen.c

@@ -79,6 +79,10 @@ PUBLIC const char __driConfigOptions[] =
 	 DRI_CONF_DESC(en, "Enable early Z in classic mode (unstable, 945-only).")
       DRI_CONF_OPT_END
 
+      DRI_CONF_OPT_BEGIN(fragment_shader, bool, false)
+	 DRI_CONF_DESC(en, "Enable limited ARB_fragment_shader support on 915/945.")
+      DRI_CONF_OPT_END
+
    DRI_CONF_SECTION_END
    DRI_CONF_SECTION_QUALITY
       DRI_CONF_FORCE_S3TC_ENABLE(false)
@@ -91,7 +95,7 @@ PUBLIC const char __driConfigOptions[] =
    DRI_CONF_SECTION_END
 DRI_CONF_END;
 
-const GLuint __driNConfigOptions = 10;
+const GLuint __driNConfigOptions = 11;
 
 #ifdef USE_NEW_INTERFACE
 static PFNGLXCREATECONTEXTMODES create_context_modes = NULL;