templatematch: mark pads as proxy caps

Allows negotiation to happen properly
opencv templatematch: Set caps to BGR order

templatematch operates on BGR data. In fact, OpenCV's IplImage always
stores color image data in BGR order -- this isn't documented at all in
the OpenCV source code, but there are hints around the web (see for
example
http://www.cs.iit.edu/~agam/cs512/lect-notes/opencv-intro/opencv-intro.html#SECTION00041000000000000000
and http://www.comp.leeds.ac.uk/vision/opencv/iplimage.html ).

gst_templatematch_load_template loads the template (the image to find)
from disk using OpenCV's cvLoadImage, so it is stored in an IplImage in
BGR order. But in gst_templatematch_chain, no OpenCV conversion
functions are used: the imageData pointer of the IplImage for the video
frame (the image to search in) is just set to point to the raw buffer
data. Without this fix, that raw data is in RGB order, so the call to
cvMatchTemplate ends up comparing the template's Blue channel against
the frame's Red channel, producing very poor results.
glmemory: reenable the texture_rg support for !eagl

The GST_GL_HAVE_PLATFORM_EAGL is always defined we need to compare
against the value instead.
glimagesink: keep the uploaded buffer around on successful redisplay

We might need it later to perform a redisplay.  GstGLUpload will take
of releasing the previous buffer when it receives a new buffer.

https://bugzilla.gnome.org/show_bug.cgi?id=733726