Shader build error does not give a clue which shader it was

[slipher]

In the case that a shader fails to build at the link stage, you can get an error log that doesn't tell you anything about which shader has a problem.

Warn: Link log: 
Warn: error: fragment shader input `var_TexCoords' has no matching output in the previous stage
error: fragment shader input `var_Color' has no matching output in the previous stage
 
Warn: Built-in shaders failed: Shader program failed to link! 

[slipher]

Fixed by #1927.