How to preallocate cstring for FFI call that fills it up

[scippie]

For completeness: I had to reduce the length by 1 because OpenGL returns the 
length including the 0 character, making the string contain two 0 characters at 
the end.

>From 
><https://registry.khronos.org/OpenGL-Refpages/gl4/html/glGetProgram.xhtml>:

> GL_INFO_LOG_LENGTH
> 
> params returns the number of characters in the information log for program 
> including the null termination character (i.e., the size of the character 
> buffer required to store the information log). If program has no information 
> log, a value of 0 is returned.

This is not really a big deal when simply printing the string, but if I want to 
for example query all shader uniform variables and put them in a table and then 
address them by their name, this becomes necessary because I need to add the 0 
to find them if I don't decrease the length:
    
    
      var
        numUniforms: GLint
        maxNameLen: GLint
      glGetProgramiv(result.id, GL_ACTIVE_UNIFORMS, numUniforms.addr)
      glGetProgramiv(result.id, GL_ACTIVE_UNIFORM_MAX_LENGTH, maxNameLen.addr)
      var nameBuf = newString(maxNameLen - 1)
      for i in 0.GLuint .. numUniforms.GLuint - 1:
        var
          usize: GLint
          utype: GLenum
          uloc: GLint
        glGetActiveUniform(result.id, i, maxNameLen, nil, usize.addr, 
utype.addr, nameBuf.cstring)
        uloc = glGetUniformLocation(result.id, nameBuf.cstring)
        result.uniformLocation[nameBuf] = uloc
    
    
    Run

To clarify, without the -1, I had to:
    
    
    let uColor = progTest.uniformLocation["uColor\0"]
    
    
    Run