README example Error: node lacks field: intVal
I'm trying to run the README example on macOS Sierra 10.12.6 and get this error:

```
stack trace: (most recent call last)
../../../.nimble/pkgs/nimsl-0.3/nimsl/nimsl.nim(14) getGLSLFragmentShader
../../../.nimble/pkgs/nimsl-0.3/nimsl/nimsl.nim(8) getShaderCode
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(318) genProcDef
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(384) gen
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(118) genStmtList
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(385) gen
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(180) genCall
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(391) gen
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(245) genSym
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(393) gen
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(318) genProcDef
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(384) gen
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(118) genStmtList
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(386) gen
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(195) genInfixCall
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(178) genCall
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(147) genSystemCall
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(398) gen
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(399) gen
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(349) genBracketExpr
../../../.nimble/pkgs/nimx-0.1/nimx/context.nim(251, 46) template/generic instantiation of `newCompositionWithNimsl` from here
../../../.nimble/pkgs/nimx-0.1/nimx/composition.nim(345, 41) template/generic instantiation of `getGLSLFragmentShader` from here
../../../.nimble/pkgs/nimsl-0.3/nimsl/private/glsl_codegen.nim(349, 24) Error: node lacks field: intVal
```