Portal for Windows (in steam) tries to set up compressed textures like this (from seen in apitrace)

 glTexImage2D(target = GL_TEXTURE_2D, level = 0, internalformat = GL_COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT, width = 1024, height = 1024, border = 0, format = GL_RGBA, type = GL_UNSIGNED_BYTE, pixels = NULL)
 glCompressedTexSubImage2DARB(target = GL_TEXTURE_2D, level = 0, xoffset = 0, yoffset = 0, width = 1024, height = 1024, format = GL_COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT, imageSize = 524288, data = [binary data, size = 512 kb])

The glTexImage2D would encounter an assertion failure on debug builds, and the glCompressedTexSubImage2DARB causes an division by zero on release builds. The generic Mesa core decides that S3TC compression is OK when libtxc_dxtn is installed (I have the s2tc version installed), and so texture_error_check succeeds on the first call. Nevertheless _mesa_choose_texture_format fails as S3TC is not supported on i915g. If assertions were enabled, the assertion texFormat != MESA_FORMAT_NONE would trigger, but that condition is ignored.

The block size check in glCompressedTexSubImage2DARB ("(xoffset % bw != 0) || (yoffset % bh != 0)") crashes for MESA_FORMAT_NONE, as bw and bh are zero in that case.

Uninstalling libtxc-dxtn-s2tc helps to not trigger that problem, thus the "minor" severity, as that library on a system without s3tc support is mostly pointless anyway.