GLSL: Add support for GL_EXT_texture_array

My GLSL 1.20 shaders make use of GL_EXT_texture_array (and require the extension), but glslang doesn't support/export that extension so validation of those shaders fails.

It would be useful to me if glslang supported that extension, for pre-1.30 GLSL shaders.

No file or test is named. Start by locating glslang's handling of GLSL 1.20 extensions and the validation path for required extensions; done means GL_EXT_texture_array is accepted and exported for pre-1.30 GLSL shaders without validation failure.

Written by the indexing model from the issue text.