swrast: Only avoid empty _TexEnvPrograms

If the generated shader for _TexEnvProgram is empty, force the use of
the fixed-function code.  Otherwise, go ahead and use the shader.
This works around a mysterious issue on i915 where fixed-function
software fallbacks are not working correctly.

This isn't really the fix we want, but it works around the issue.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=45872
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=45876

src/mesa/swrast/s_fragprog.c

@@ -40,7 +40,8 @@ GLboolean
 _swrast_use_fragment_program(struct gl_context *ctx)
 {
    struct gl_fragment_program *fp = ctx->FragmentProgram._Current;
-   return fp && fp != ctx->FragmentProgram._TexEnvProgram;
+   return fp && !(fp == ctx->FragmentProgram._TexEnvProgram
+                  && fp->Base.NumInstructions == 0);
 }
 
 /**