vrend: Don't fail when shader compilation fails at shader creation time

When the shader is newly created the program states may still hold old information from previous shader programs that may result in compilation failure. Don't reporte an error at this stage and just mark whether the shaders has already been compiled. Later, when the program using the shader is linked we already check whether the final vertex stage needs to be patched and recompiled, so check the other shaders as well and compile them if needed.

Fixes #114 (closed)