Support Uint to Int implicit conversions at #extension GL_ARB_gpu_shader5.

Signed-off-by: ZhiqianXia <xzq0528@outlook.com>

Test/GL_ARB_gpu_shader5.u2i.vert

@@ -0,0 +1,11 @@
+#version 150
+#extension GL_ARB_gpu_shader5 : require
+
+uniform int u1;
+uniform int u2;
+out   vec4 result;
+void main()
+{
+    uint v = 0;
+    v = uint(u2) - u1;  // implicit conversions
+}