add WEBP_DSP_INIT / WEBP_DSP_INIT_FUNC

this internalizes the init checks and provides stronger synchronization
with pthreads when available while still allowing VP8GetCPUInfo to be
modified (mostly for testing purposes). windows is left as is since a
critical section or mutex would cause a leak.

Change-Id: Ieb997e014f2805c0ae39c16f13337663521356f4
(cherry picked from commit d77bf512bd0c4ff53019e357a2d24f6c4e3fbefc)

• src/dec/vp8_dec.c[diff]
• src/dsp/alpha_processing.c[diff]
• src/dsp/cost.c[diff]
• src/dsp/dec.c[diff]
• src/dsp/dsp.h[diff]
• src/dsp/enc.c[diff]
• src/dsp/filters.c[diff]
• src/dsp/lossless.c[diff]
• src/dsp/lossless_enc.c[diff]
• src/dsp/rescaler.c[diff]
• src/dsp/ssim.c[diff]
• src/dsp/upsampling.c[diff]
• src/dsp/yuv.c[diff]
• src/enc/near_lossless_enc.c[diff]
• src/enc/picture_csp_enc.c[diff]