[Piglit] problem with msaa dual source blending tests

[Vadim Girlin]

Hi,

It seems there is a problem with alpha-to-coverage-dual-src-blend test.

AFAICS the test uses DrawPixels with dual source blending and
fixed-function fragment pipeline. According to GL and
ARB_blend_func_extended specs:

"Rendering using any of the blend functions that consume the second
input color (SRC1_COLOR, ONE_MINUS_SRC1_COLOR, SRC1_ALPHA or
ONE_MINUS_SRC1_ALPHA) using fixed function will produce undefined
results."

So it looks like the test relies on the undefined behaviour. Here is the
part of the stack for that DrawPixels call:

	12 visualize_image() common.cpp:1775
	11 draw_image_to_window_system_fb() draw-buffers-common.cpp:704
	10 draw_reference_image() draw-buffers-common.cpp:823
	9  piglit_display() alpha-to-coverage-dual-src-blend.cpp:104

Probably blending should be temporarily disabled in that case.

Vadim