guest glTransformFeedbackVaryings doesn't work on pure GLES backend
Currently at virglrenderer side, it uses gl_NextBuffer on host side to implement glTransformFeedbackVaryings from guest. The issue here is, GLES doesn't support gl_NextBuffer. So this is broken on a pure GLES backend.
Example:
Guest is calling glTransformFeedbackVaryings with buffer mode GL_SEPARATE_ATTRIBS with variables like {"v1", "v2"}; this finally "translated" to glTransformFeedbackVaryings with buffer mode GL_INTERLEAVED_ATTRIBS at host side with variables like {"v1", "gl_NextBuffer", "v2"} and isn't supported by GLES