Misuse of GL_BGRA as internal format in TexImage2D call with only GL_EXT_BGRA available

When running random QtQuick programs under Mesa 10.2.4 (radeonsi on a Radeon 7970), large number of elements are all black. Running the program with Mesa dumping debug information, the following is printed (sddm-greeter in this case):
Mesa: User error: GL_INVALID_VALUE in glTexImage2D(internalFormat=GL_BGRA)
Mesa: User error: GL_INVALID_VALUE in glTexImage2D(internalFormat=GL_BGRA)
[00:46:12.322] (II) GREETER: QSGTextureAtlas: texture atlas allocation failed, code=501

Investigating further, in qsgatlastexture.cpp on line 166, the if condition stopping the internal format from being set to GL_BGRA is satisfied by the GL_EXT_bgra extension being present (which it is). However, reading the specification (pulled from https://www.opengl.org/registry/specs/EXT/bgra.txt ), it appears that this is not sufficient to show that the internal format can be set to GL_BGRA. Commenting out the assignment line does indeed fix the problem, with the error message disappearing from sddm-greeter's output, and visual are rendered as expected.

Note other random QtQuick programs are KWin and Plasma from the Plasma 5.0 release. I haven't tried running them with the quick fix yet (reporting this bug first). If they don't get fixed, I'll report my further findings.