glcolorconvert: Ensure glcolorconvert does not miss supported RGB formats

Issue

gst-launch-1.0 videotestsrc num-buffers=10 ! video/x-raw,format=NV12,width=1920,height=1080 ! glupload ! glcolorconvert ! "video/x-raw(memory:GLMemory),format=RGB16,texture-target=2D" ! gldownload ! filesink location=./test.rgb

When using gstreamer 1.24.0, the above pipeline will fail with the following error log:

0:00:00.088643768  7026 0xaaaad15a43e0 ERROR           GST_PIPELINE gst/parse/grammar.y:1128:gst_parse_perform_link: could not link glcolorconvertelement0 to gldownloadelement0, glcolorconvertelement0 can't handle caps video/x-raw(memory:GLMemory), format=(string)RGB16, texture-target=(string)2D
WARNING: erroneous pipeline: could not link glcolorconvertelement0 to gldownloadelement0, glcolorconvertelement0 
can't handle caps video/x-raw(memory:GLMemory), format=(string)RGB16, texture-target=(string)2D

troubleshoot

This is because when calculating the transformable supported RGB formats of glcolorconvert, some RGB formats, such as RGB16, may be omitted. The problematic code is as follows:

gst_gl_color_convert_caps_transform_format_info (GstGLContext * context,
    gboolean output, GstCaps * caps)
{
  _init_value_string_list (&rgb_formats, "RGBA", "ARGB", "BGRA", "ABGR", "RGBx",
      "xRGB", "BGRx", "xBGR", "RGB", "BGR", "ARGB64", "BGR10A2_LE",
      "RGB10A2_LE", "RGBA64_LE", "RGBA64_BE", "RBGA", NULL);
  _init_supported_formats (context, output, &supported_formats);
  gst_value_intersect (&supported_rgb_formats, &rgb_formats,
      &supported_formats);
  ... ...
}

glcolorconvert can only support outputting RGB16 format under specific conditions, so rgd_formats does not have RGB16. Due to the fact that supported_rgb_formats are the intersection of rgb_formats and supported_rgb_formats, supported_rgb formats may miss the RGB16 format.

Solution

Add 'RGB16' and other RGB-related formats to rgd_formats list.