Added `textureCube` macro for GLSL >= 130. (#2728)

`textureCube` requires extension `GL_NV_shadow_samplers_cube`, however `texture` can sample a cube sampler without the extension.

src/bgfx_shader.sh

@@ -608,6 +608,7 @@ vec4  mod(vec4  _a, vec4  _b) { return _a - _b * floor(_a / _b); }
 #		define texture2D(_sampler, _coord)      texture(_sampler, _coord)
 #		define texture2DArray(_sampler, _coord) texture(_sampler, _coord)
 #		define texture3D(_sampler, _coord)      texture(_sampler, _coord)
+#		define textureCube(_sampler, _coord)    texture(_sampler, _coord)
 #		define texture2DLod(_sampler, _coord, _lod)                textureLod(_sampler, _coord, _lod)
 #		define texture2DLodOffset(_sampler, _coord, _lod, _offset) textureLodOffset(_sampler, _coord, _lod, _offset)
 #	endif // BGFX_SHADER_LANGUAGE_GLSL >= 130