glsl: Slightly change the semantic of _LinkedShaders

Previously _LinkedShaders was a compact array of the linked shaders
for each shader stage.  Now it is arranged such that each slot,
indexed by the MESA_SHADER_* defines, refers to a specific shader
stage.  As a result, some slots will be NULL.  This makes things a
little more complex in the linker, but it simplifies things in other
places.

As a side effect _NumLinkedShaders is removed.

NOTE: This may be a candidate for the 7.9 branch.  If there are other
patches that get backported to 7.9 that use _LinkedShader, this patch
should be cherry picked also.
diff --git a/src/glsl/main.cpp b/src/glsl/main.cpp
index 9350592..08a44c9 100644
--- a/src/glsl/main.cpp
+++ b/src/glsl/main.cpp
@@ -40,8 +40,19 @@
 extern "C" struct gl_shader *
 _mesa_new_shader(struct gl_context *ctx, GLuint name, GLenum type);
 
+extern "C" void
+_mesa_reference_shader(struct gl_context *ctx, struct gl_shader **ptr,
+                       struct gl_shader *sh);
+
 /* Copied from shader_api.c for the stand-alone compiler.
  */
+void
+_mesa_reference_shader(struct gl_context *ctx, struct gl_shader **ptr,
+                       struct gl_shader *sh)
+{
+   *ptr = sh;
+}
+
 struct gl_shader *
 _mesa_new_shader(struct gl_context *ctx, GLuint name, GLenum type)
 {
@@ -319,7 +330,7 @@
 	 printf("Info log for linking:\n%s\n", whole_program->InfoLog);
    }
 
-   for (unsigned i = 0; i < whole_program->_NumLinkedShaders; i++)
+   for (unsigned i = 0; i < MESA_SHADER_TYPES; i++)
       talloc_free(whole_program->_LinkedShaders[i]);
 
    talloc_free(whole_program);