A macro for disabling the shader cache

An commercial user is requesting adding a macro to forcibly disabling the shader cache, that is shared by QOpenGLShaderProgram and QRhi OpenGL backend.

The back ground of this request :

• QOpenGLProgramBinarySupportCheck::QOpenGLProgramBinarySupportCheck() checks :
   (Check 1) OpenGLES version is 3 or newer? If YES, it sets the shader cache as "supported".
   (Check 2) Even if OpenGLES is older than 3, is there "GL_OES_get_program_binary" extension? If YES, it sets the shader cache as "supported".

• However, under some Yocto Linux environment, when the "GL_OES_get_program_binary" extension is found in Check 2 and the shader cache gets enabled, calling glGetProgramBinary() causes a heap-buffer-overflow detected by Address Sanitizer. This can be avoided by forcibly disabling the shader cache, i.e. fixedly setting "const bool hasExt = false;"

• To resove this issue, the user is requesting a macro switch to disable the shader cache on demand.