GLSL code generation does not detect reserved name nor integer constant usage

[kwokcb]

Issue

The GLSL code generator will not recognize that

• '__' anywhere in the node name, or
• node names which start with numbers
  are invalid.

Each generator is supposed to patch invalid names but these are currently not detected.

Example

<?xml version="1.0"?>
<materialx version="1.39">
  <add name="12" type="color3" xpos="10.173913" ypos="-3.275862" />
  <surfacematerial name="surfacematerial" type="material" xpos="17.710144" ypos="-1.258621">
    <input name="surfaceshader" type="surfaceshader" nodename="standard_surface_surfaceshader" />
  </surfacematerial>
  <standard_surface name="standard_surface_surfaceshader" type="surfaceshader" xpos="13.050725" ypos="-7.758621">
    <input name="base_color" type="color3" nodename="12" />
    <input name="specular_color" type="color3" nodename="a__" />
  </standard_surface>
  <add name="a__" type="color3" xpos="10.376812" ypos="-6.172414" />
</materialx>

Errors (can load into desktop viewer or node editor:

Error in compiling fragment shader:
0(27) : error C0000: syntax error, unexpected integer constant, expecting ';' at token "<int-const>"
0(28) : error C0000: syntax error, unexpected integer constant, expecting ';' at token "<int-const>"
0(29) : error C7528: OpenGL reserves names containing '__'
0(30) : error C7528: OpenGL reserves names containing '__'
0(1902) : error C0000: syntax error, unexpected integer constant, expecting ';' or '(' at token "<int-const>"
0(1903) : error C7528: OpenGL reserves names containing '__'
0(1905) : error C0000: syntax error, unexpected identifier, expecting ')' at token "_out"

This was created by editing node names in the node editor.