avoid1BitAlphaTextureFormats workaround was added to angle in 288584
due to an old driver bug on Intel and AMD that 1-bit alpha always
rounds up when converting from float to unsigned int. The workaround
uses rgba8 instead of *a1 to avoid the driver bug and optimizes precision.
However, this workaround brings a new issue because 1-bit alpha can only
represent u0 or u1 while 8 bits alpha covers from u0 to u255. For
example, if we expect to render to a rgb5_a1 renderbuffer, the expected
alpha value should be 0 or 255, but actually get a value between 0 and
255 which is incorrect. The current Intel drivers have fixed the old
driver bug. So we suggest to disable this workaround for Intel drivers.

TEST=dEQP_GLES2.Default/functional_fbo_render_color_clear_rbo_rgb5_a1_depth_component16_stencil_index8
--deqp-egl-display-type=angle-gl
BUG=angleproject:2349

Change-Id: I14933f92fa27031ff7442fa437f77a3c67f2f1db
Reviewed-on: https://chromium-review.googlesource.com/1034163Reviewed-by: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>