glsl/linker: Fix xfb with explicit locations and 64bit types

1) Per GL_ARB_enhanced_layouts if explicit location is set for varying,
each struct member, array element and matrix row will take
separate location. With GL_ARB_gpu_shader_fp64/GL_ARB_gpu_shader_int64
they may take two locations.

Examples:

| layout(location=0) dvec3[2] a; | layout(location=4) vec2[4] b; |
|                                |                               |
|   32b 32b 32b 32b              |   32b 32b 32b 32b             |
| 0  X   X   Y   Y               | 4  X   Y   0   0              |
| 1  Z   Z   0   0               | 5  X   Y   0   0              |
| 2  X   X   Y   Y               | 6  X   Y   0   0              |
| 3  Z   Z   0   0               | 7  X   Y   0   0              |

Previously it wasn't taken into account.

2) Captured double-precision variables should be aligned to
8 bytes per GL_ARB_gpu_shader_fp64:
 "If any variable captured in transform feedback has double-precision
 components, the practical requirements for defined behavior are:
     ...
 (c) each double-precision variable captured must be aligned to a
     multiple of eight bytes relative to the beginning of a vertex."

v2: fix `output_size` calculations
         ( Andrii Simiklit <andrii.simiklit@globallogic.com> )

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/1667
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/2333>

src/compiler/glsl/link_varyings.h

@@ -77,10 +77,16 @@ struct tfeedback_candidate
    const glsl_type *type;
 
    /**
-    * Offset within the toplevel variable where this varying occurs (counted
-    * in multiples of the size of a float).
+    * Offset within the toplevel variable where this varying occurs.
+    * Counted in floats.
     */
-   unsigned offset;
+   unsigned struct_offset_floats;
+
+   /**
+    * Offset within the xfb with respect to alignment requirements.
+    * Counted in floats.
+    */
+   unsigned xfb_offset_floats;
 };