shaderc_glsl: Fix error when mixing SamplerArray and `flat` keyword

Use of the `flat` keyword will have `glslopt_optimize` run with glsl
version 130. In this mode, it will change calls of `texture2DArray` to
the more general `textureArray`. However, this call results in an error
when reading the compiled shader:

```
bgfx/src/renderer_gl.cpp:5904: BGFX Failed to compile shader.
0: 0(45) : error C1503: undefined variable "textureArray"
```
Defining this call to be `texture` avoids this error.

src/renderer_gl.cpp

@@ -5660,6 +5660,7 @@ namespace bgfx { namespace gl
 						else
 						{
 							bx::write(&writer, "#define texture2DArrayLodEXT texture2DArrayLod\n");
+							bx::write(&writer, "#define textureArray texture\n");
 						}
 					}