mesa: change 'SHADER_SUBST' facility to work with env variables

Patch modifies existing shader source and replace functionality to work
with environment variables rather than enable dumping on compile time.
Also instead of _mesa_str_checksum, _mesa_sha1_compute is used to avoid
collisions.

Functionality is controlled via two environment variables:

MESA_SHADER_DUMP_PATH - path where shader sources are dumped
MESA_SHADER_READ_PATH - path where replacement shaders are read

v2: cleanups, add strerror if fopen fails, put all functionality
    inside HAVE_SHA1 since sha1 is required

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Suggested-by: Eero Tamminen <eero.t.tamminen@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>

docs/shading.html

@@ -63,6 +63,20 @@ execution.  These are generally used for debugging.
 Example:  export MESA_GLSL=dump,nopt
 </p>
 
+<p>
+Shaders can be dumped and replaced on runtime for debugging purposes. Mesa 
+needs to be configured with '--with-sha1' to enable this functionality. This 
+feature is not currently supported by SCons build.
+
+This is controlled via following environment variables:
+<ul>
+<li><b>MESA_SHADER_DUMP_PATH</b> - path where shader sources are dumped
+<li><b>MESA_SHADER_READ_PATH</b> - path where replacement shaders are read
+</ul>
+Note, path set must exist before running for dumping or replacing to work. 
+When both are set, these paths should be different so the dumped shaders do 
+not clobber the replacement shaders.
+</p>
 
 <h2 id="support">GLSL Version</h2>