mesa/shaderapi: add an optional shader override mechanism

MESA_SHADER_READ_PATH is handy but it's not usable in
all cases.

This commit allows to implement an alternative mechanism
without assuming too much about how it's done, nor where/how
the shaders are stored.

When this is enabled MESA_SHADER_DUMP_PATH,
MESA_SHADER_CAPTURE_PATH and MESA_GLSL env var handling is
disabled.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/11621>

meson_options.txt

@@ -502,3 +502,9 @@ option(
   choices : ['auto', 'panfrost', 'intel', 'freedreno'],
   description: 'List of Perfetto datasources to build. If this is set to `auto`, datasources that can not be build are skipped. Default: [`auto`]'
 )
+option(
+  'custom-shader-replacement',
+  type : 'string',
+  value : '',
+  description : 'Enable a custom shader replacement mechanism. Note that enabling this option requires adding/generating a shader_replacement.h file that can be included (see shaderapi.c).'
+)