cli: prefer detected python executable instead of harcoded one

In ArchLinux, python means python3

v2: Don't change Windows behavior -- Jose.
glretrace: Don't dump null images when there are errors reading the framebuffer.

Issue #284.
glretrace: Skip incomplete framebuffers when dumping state.

Issue #284.
retrace: Assert image is never null.
specs: Merge dxgi1_2.py into dxgi.py.
specs: Tweak d3d1[01]sdklayers.py
dxgitrace: Try to detect non-overlapping buffer writes.

Work in progress.
dxgitrace: Shadow D3D11 buffers properly.
dxgitrace: Fill a pattern when discarding and shadowing.
dxgitrace: Ensure buffers are initialized.
d3dretrace: Force windowed mode on IDXGISwapChain::SetFullscreenState too.
cmake: Enable SSE2 intrinsics on x86.
dxgitrace: Use CRC32C hashes of 512byte blocks instead of memcopies.

Much faster.