glcolorconvert: fix non-passthrough of RGB -> RGB

On i.MX we're always drawing EGLImages once since textures
are imported from dma-bufs that can't be re-exported.
When input and output are both RGB GStreamer tries to compile
a shader for the purpose, but the alpha channel mapping is
missing when

- input is same format as output
- that same format is only 3 chars

The channel map must always be 4 chars long, one char for
each of R G B A.

Bug: 161191133
Change-Id: Ib8d19fb30f09ef8ff24260014a05bb24e0a13b44
1 file changed