st/mesa: optimize 4-component ubyte glDrawPixels

st/mesa: optimize 4-component ubyte glDrawPixels

If we didn't find a gallium surface format that exactly matched the
glDrawPixels format/type combination, we used some other 32-bit packed
RGBA format and swizzled the whole image in the mesa texstore/format code.

That slow path can be avoided in some common cases by using the
pipe_samper_view's swizzle terms to do the swizzling at texture sampling
time instead.

For now, only GL_RGBA/ubyte and GL_BGRA/ubyte combinations are supported.
In the future other formats and types like GL_UNSIGNED_INT_8_8_8_8 could
be added.

v2: fix incorrect swizzle setup (need to invert the tex format's swizzle)

Reviewed by: Jose Fonseca <jfonseca@vmware.com>