The implementation of OpenGLES_v2 glCompressedTexSubImage2D() API
lacks of a sanity check for compressed formats. When a
non-compressed format is specified (e.g. GL_RGB), an unreachable
condition of SS's internal function, ComputeCompressedSize(), is
hit.

This patch is to add a check in CompressedTexSubImage2D() function
to prevent invalid formatd from being entered, in terms of
OpenGL_v2 API integrity in accordance with Khorons's specification

Bug: b/116776984
Test: Manual tests using OGLESHelloAPI by specifying GL_RGB format
Change-Id: Icc964ecb9dbbdef6c6bc82dab42c35290917159e
Reviewed-on: https://swiftshader-review.googlesource.com/c/21548Reviewed-by: Merck Hung <merckhung@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Tested-by: Merck Hung <merckhung@google.com>