Dongwon Kim authored Mar 26, 2018 and Matt Turner committed Jun 18, 2018

Optional binding of variables can be processed before linking shader
objects for creating shader program. It is activated by adding lines
with a keyword "BindAttribLoc" followed by name and index as,

"BindAttribLoc name_str1 <index1>"

For example,

[require]
......
BindAttrbLoc vertex 1
BindAttrbLoc coord 2
BindAttrbLoc col 3

This makes the shader-db run

glBindAttribLocation(p, 1, "vertex");
glBindAttribLocation(p, 2, "coord");
glBindAttribLocation(p, 3, "col");

before glLinkProgram() to include these binding info in binary shader
program.

v2: get_shaders returns its own head of list for binding variables
    instead of using a global head to support parallel processing of
    multiple shader_tests

Signed-off-by: Dongwon Kim <dongwon.kim@intel.com>