glsl/cache: save and restore ExternalSamplersUsed

Shaders that need special code for external samplers were broken if
they were loaded from the cache.

Cc: 18.1 <mesa-stable@lists.freedesktop.org>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>

src/compiler/glsl/serialize.cpp

@@ -1044,6 +1044,7 @@ write_shader_metadata(struct blob *metadata, gl_linked_shader *shader)
    blob_write_bytes(metadata, glprog->sh.SamplerTargets,
                     sizeof(glprog->sh.SamplerTargets));
    blob_write_uint32(metadata, glprog->ShadowSamplers);
+   blob_write_uint32(metadata, glprog->ExternalSamplersUsed);
 
    blob_write_bytes(metadata, glprog->sh.ImageAccess,
                     sizeof(glprog->sh.ImageAccess));
@@ -1096,6 +1097,7 @@ read_shader_metadata(struct blob_reader *metadata,
    blob_copy_bytes(metadata, (uint8_t *) glprog->sh.SamplerTargets,
                    sizeof(glprog->sh.SamplerTargets));
    glprog->ShadowSamplers = blob_read_uint32(metadata);
+   glprog->ExternalSamplersUsed = blob_read_uint32(metadata);
 
    blob_copy_bytes(metadata, (uint8_t *) glprog->sh.ImageAccess,
                    sizeof(glprog->sh.ImageAccess));