vo_opengl: default scaler-resizes-only sub-option to yes

Often requested. The main argument, that prominent scalers like sharpen
change the image even if no scaling happens, disappeared anyway.
("sharpen", unsharp masking, is neither prominent nor a scaler anymore.
This is an artifact from MPlayer, which fuses unsharp masking with
bilinear scaling in order to make it single-pass, or so.)

DOCS/interface-changes.rst

@@ -27,6 +27,7 @@ Interface changes
       the old default)
     - VO opengl custom shaders must now use "sample_pixel" as function name,
       instead of "sample"
+    - change VO opengl scaler-resizes-only default to enabled
  --- mpv 0.15.0 ---
     - change "yadif" video filter defaults
  --- mpv 0.14.0 ---

DOCS/man/vo.rst

@@ -444,7 +444,7 @@ Available video output drivers are:
         Disable the scaler if the video image is not resized. In that case,
         ``bilinear`` is used instead whatever is set with ``scale``. Bilinear
         will reproduce the source image perfectly if no scaling is performed.
-        Note that this option never affects ``cscale``.
+        Enabled by default. Note that this option never affects ``cscale``.
 
     ``pbo``
         Enable use of PBOs. On some drivers this can be faster, especially if