gl_shader_decompiler: Initialize gl_Position on vertex shaders

src/video_core/renderer_opengl/gl_shader_decompiler.cpp

@@ -419,6 +419,10 @@ public:
         code.AddLine("void main() {{");
         ++code.scope;
 
+        if (stage == ShaderType::Vertex) {
+            code.AddLine("gl_Position = vec4(0.0f, 0.0f, 0.0f, 1.0f);");
+        }
+
         if (ir.IsDecompiled()) {
             DecompileAST();
         } else {