cogl: Pick glReadPixels format by target, not source

Presumably glReadPixels itself can be more performant with pixel format
conversions than doing a fix-up conversion on the CPU afterwards. Hence,
pick required_format based on the destination rather than the source, so
that it has a better chance to avoid the fix-up conversion.

With CoglOnscreen objects, CoglFramebuffer::internal_format (the source
format) is also wrong. It is left to a default value and never set to
reflect the reality. In other words, read-pixels had an arbitrary
intermediate pixel format that was used in glReadPixels and then fix-up
conversion made it work for the destination.

The render buffers (GBM surface) are allocated as DRM_FORMAT_XRGB8888.
If the destination buffer is allocated as the same format, the Cogl
read-pixels first converts with glReadPixels XRGB -> ABGR because of the
above default format, and then the fix-up conversion does ABGR -> XRGB.
This case was observed with DisplayLink outputs, where the native
renderer must use the CPU copy path to fill the "secondary GPU"
framebuffers.

This patch stops using internal_format and uses the desired destination
format instead.

_cogl_framebuffer_gl_read_pixels_into_bitmap() will still use
internal_format to determine alpha premultiplication state and multiply
or un-multiply as needed. Luckily all the formats involved in the
DisplayLink use case are always _PRE and so is the default
internal_format too, so things work in practise.

Furthermore, the GL texture_swizzle extension can never apply to
glReadPixels. Not even with FBOs, as found in this discussion:
#72
Therefore the target_format argument is hardcoded to something that can
never match anything, which will prevent the swizzle from being assumed.

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