gl: download: Fix the wrong transformed result from src direction in transform_caps().

gl: download: Fix the wrong transformed result from src direction in transform_caps().

The current manner in transform_caps() for src direction is not very correct. For example,
when the src caps is:
  video/x-raw(memory:DMABuf); video/x-raw; video/x-raw(memory:GLMemory)
this function returns:
  video/x-raw(memory:DMABuf); video/x-raw; video/x-raw(memory:GLMemory)
as the sink caps. This is not correct, because DMABuf feature is not even in the sink pad's
caps template. The correct answer should be:
  video/x-raw(memory:GLMemory); video/x-raw
only.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/1058>