Make a number of key optimizations

- Changed glMapBuffer to glMapBufferRange to allow invalidation.  Using
   just glMapBuffer alone was causing some unacceptable stalls.

 - Changed dynamic buffers from GL_DYNAMIC_WRITE to GL_STREAM_WRITE
   because I had misunderstood the OpenGL specification

 - Added _OPENGL and _D3D11 builtin preprocessor macros to effects to
   allow special processing if needed

 - Added fmod support to shaders (NOTE: D3D and GL do not function
   identically with negative numbers when using this.  Positive numbers
   however function identically)

 - Created a planar conversion shader that converts from packed YUV to
   planar 420 right on the GPU without any CPU processing.  Reduces
   required GPU download size to approximately 37.5% of its normal rate
   as well.  GPU usage down by 10 entire percentage points despite the
   extra required pass.

libobs-opengl/gl-subsystem.c

@@ -163,6 +163,11 @@ void convert_sampler_info(struct gs_sampler_state *sampler,
 	               info->max_anisotropy, sampler->max_anisotropy);
 }
 
+const char *device_preprocessor_name(void)
+{
+	return "_OPENGL";
+}
+
 device_t device_create(struct gs_init_data *info)
 {
 	struct gs_device *device = bzalloc(sizeof(struct gs_device));