Disable implementations mentioned in the PIXMAN_DISABLE environment variable.

Disable implementations mentioned in the PIXMAN_DISABLE environment variable.

With this, it becomes possible to do

     PIXMAN_DISABLE="sse2 mmx" some_app

which will run some_app without SSE2 and MMX enabled. This is useful
for benchmarking, testing and narrowing down bugs.

The current list of implementations that can be disabled:

    fast
    mmx
    sse2
    arm-simd
    arm-iwmmxt
    arm-neon
    mips-dspr2
    vmx

The general and noop implementations can't be disabled because pixman
depends on those being available for correct operation.

Reviewed-by: Matt Turner <mattst88@gmail.com>