render GBRP10 in opengl client

[Antoine Martin]

Split from #1462

[Antoine Martin]

Issues:

• opengl rendering is probably OK, but it would be nice to do the bitshift using the shader (tweak RGBP2RGB_SHADER?) - see ​Colour bit-wise shift in OpenGL shader GLSL
• first frame looks OK, but then everything goes garbled

[Antoine Martin]

almost works

[Antoine Martin]

Pretty much working as of r26945, just needs:

• 10-bit windows sometimes render with the same repeated pattern, 4 times over - some alignment problem?
• the byte-swapping code is super-slow, as it's done using a bytearray in python - should use cython and read / write longs at a time?
• window icons? (also use 30-bit?)

[Antoine Martin]

The official OpenGL documentation on ​https://www.khronos.org/opengl/wiki/Pixel_Transfer says ​glPixelStore: GL_UNPACK_SWAP_BYTES has no effect on the memory order of components within a pixel, only on the order of bytes within components or indices.
It wasn't clear to me that this would do the byteswap for GL_UNSIGNED_SHORT, but it does. Problem solved!
r26948 replaces the slow byte-swapping code with glPixelStorei(GL_UNPACK_SWAP_BYTES, True) for BGR48 only.

[Antoine Martin]

For non-opengl client, see #2839.

[Antoine Martin]

more correct handling of 10-bit data (hacked: free memleak included)

[Antoine Martin]

Correctly fixed in r26960: we have to use a csc step to convert to r210.

It might be possible to bit-shift GBRP10 to GBRP16 using a shader, but this is beyond me.