glsl/linker: always validate explicit location among inputs

Outputs are always validated when having explicit locations and we
were trusting its outcome to catch similar problems with the inputs
since, in case of having undefined outputs for existing inputs, we
would be already reporting a linker error.

However, consider this case:

  " Shader stage n:
    ---------------

    ...

    layout(location = 0) out float a;

    ...

    Shader stage n+1:
    -----------------

    ...

    layout(location = 0) in float b;
    layout(location = 0) in float c;

    ...
  "

Currently, this won't report a linker error even though location
aliasing is happening for the inputs.

Therefore, we also need to validate the inputs independently from the
outcome of the outputs validation.

Cc: Timothy Arceri <tarceri@itsqueeze.com>
Cc: Iago Toral Quiroga <itoral@igalia.com>
Cc: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Andres Gomez <agomez@igalia.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/compiler/glsl/link_varyings.cpp

@@ -687,7 +687,8 @@ cross_validate_outputs_to_inputs(struct gl_context *ctx,
                                  gl_linked_shader *consumer)
 {
    glsl_symbol_table parameters;
-   struct explicit_location_info explicit_locations[MAX_VARYING][4] = { 0 };
+   struct explicit_location_info output_explicit_locations[MAX_VARYING][4] = { 0 };
+   struct explicit_location_info input_explicit_locations[MAX_VARYING][4] = { 0 };
 
    /* Find all shader outputs in the "producer" stage.
     */
@@ -705,7 +706,7 @@ cross_validate_outputs_to_inputs(struct gl_context *ctx,
           * differently because they do not need to have matching names.
           */
          if (!validate_explicit_variable_location(ctx,
-                                                  explicit_locations,
+                                                  output_explicit_locations,
                                                   var, prog, producer)) {
             return;
          }
@@ -763,6 +764,12 @@ cross_validate_outputs_to_inputs(struct gl_context *ctx,
                compute_variable_location_slot(input, consumer->Stage);
             unsigned slot_limit = idx + num_elements;
 
+            if (!validate_explicit_variable_location(ctx,
+                                                     input_explicit_locations,
+                                                     input, prog, consumer)) {
+               return;
+            }
+
             while (idx < slot_limit) {
                if (idx >= MAX_VARYING) {
                   linker_error(prog,
@@ -771,7 +778,7 @@ cross_validate_outputs_to_inputs(struct gl_context *ctx,
                   return;
                }
 
-               output = explicit_locations[idx][input->data.location_frac].var;
+               output = output_explicit_locations[idx][input->data.location_frac].var;
 
                if (output == NULL ||
                    input->data.location != output->data.location) {