Support both signed and unsigned integer type incomplete textures.

Bug: angleproject:5502
Bug: angleproject:4432
Tests: IncompleteTextureTestES3.*IntegerType*
       dEQP.KHR_GLES31/core_sample_variables_mask_rgba8*i_*
Change-Id: Ic8c972aac0ca8589b26333b66dd0cc5fb5134043
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2613245
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>