glsl: validate global out xfb_stride qualifiers and set stride on empty buffers

Here we use the built-in validation in
ast_layout_expression::process_qualifier_constant() to check for mismatching
global out strides on buffers in a single shader.

From the ARB_enhanced_layouts spec:

   "While *xfb_stride* can be declared multiple times for the same buffer,
   it is a compile-time or link-time error to have different values
   specified for the stride for the same buffer."

For intrastage validation a new helper link_xfb_stride_layout_qualifiers()
is created. We also take this opportunity to make sure stride is at least
a multiple of 4, we will validate doubles at a later stage.

From the ARB_enhanced_layouts spec:

   "If the buffer is capturing any double-typed outputs, the stride must
   be a multiple of 8, otherwise it must be a multiple of 4, or a
   compile-time or link-time error results."

Finally we update store_tfeedback_info() to apply the strides to
LinkedTransformFeedback and update the buffers bitmask to mark any global
buffers with a stride as active. For example a shader with:

layout (xfb_buffer = 0, xfb_offset = 0)  out vec4 gs_fs;
layout (xfb_buffer = 1, xfb_stride = 64) out;

Is expected to have a buffer bound to both 0 and 1.

From the ARB_enhanced_layouts spec:

   "A binding point requires a bound buffer object if and only if its
   associated stride in the program object used for transform feedback
   primitive capture is non-zero."

Reviewed-by: Dave Airlie <airlied@redhat.com>

src/compiler/glsl/link_varyings.cpp

@@ -930,6 +930,17 @@ store_tfeedback_info(struct gl_context *ctx, struct gl_shader_program *prog,
       unsigned buffer =
          num_tfeedback_decls ? tfeedback_decls[0].get_buffer() : 0;
 
+      /* Apply any xfb_stride global qualifiers */
+      if (has_xfb_qualifiers) {
+         for (unsigned j = 0; j < MAX_FEEDBACK_BUFFERS; j++) {
+            if (prog->TransformFeedback.BufferStride[j]) {
+               buffers |= 1 << j;
+               prog->LinkedTransformFeedback.Buffers[j].Stride =
+                  prog->TransformFeedback.BufferStride[j] / 4;
+            }
+         }
+      }
+
       for (unsigned i = 0; i < num_tfeedback_decls; ++i) {
          if (tfeedback_decls[i].is_next_buffer_separator()) {
             num_buffers++;