gl: initial support for texture targets other than GL_TEXTURE_2D

gl: initial support for texture targets other than GL_TEXTURE_2D

Make GstGLMemory hold the texture target (tex_target) the texture it represents
(tex_id) is bound to. Modify gst_gl_memory_wrapped_texture and
gst_gl_download_perform_with_data to take the texture target as an argument.

This change is needed to support wrapping textures created outside libgstgl,
which might be bound to a target other than GL_TEXTURE_2D. For example on OSX
textures coming from VideoToolbox have target GL_TEXTURE_RECTANGLE.

With this change we still keep (and sometimes imply) GL_TEXTURE_2D as the
target of textures created with libgstgl.

API: modify GstGLMemory
API: modify gst_gl_memory_wrapped_texture
API: gst_gl_download_perform_with_data