glsl/nir: add glsl_types::explicit_size plus nir C wrapper

glsl/nir: add glsl_types::explicit_size plus nir C wrapper

While using SPIR-V shaders (ARB_gl_spirv), layout data is not implicit
to a specific value (std140, std430, etc) but explicitly included on
the type (explicit values for offset, stride and row_major).

So this method is equivalent to the existing std140_size and
std430_size, but using such explicit values.

Note that the value returned by this method is only valid if such data
is set, so when dealing with SPIR-V shaders.

v2: (all changes suggested by Jason Ekstrand)
   * Iterate through all struct members, instead of assume that fields
     are ordered by offset
   * Use else if
   * Take into account the case that explicit_stride > elem_size, to
     fine graine the final size on arrays and matrices
   * Handle different bit-sizes in general, not just 32 and 64.

v3: (change suggested by Caio Marcelo de Oliveira Filho)
   * fix up explicit_size() to consider interface types

Signed-off-by: Alejandro Piñeiro <apinheiro@igalia.com>
Signed-off-by: Antia Puentes <apuentes@igalia.com>
Signed-off-by: Neil Roberts <nroberts@igalia.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>