Use the magic behaviour of GL_BACK in GLES 1 and 2 as well as 3

In GLES 3 it is not possible to select rendering to the front buffer and
instead selecting GL_BACK has the magic interpretation that it is either the
front buffer on single-buffered configs or the back buffer on double-buffered.
GLES 1 and 2 have no way of selecting the draw buffer at all. In that case we
were initialising the draw buffer to either GL_FRONT or GL_BACK depending on
the context's config and then leaving it at that.

When we switch to having configless contexts we ideally want Mesa to
automatically switch between the front and back buffer whenever a double-
or single-buffered surface is bound. To make this happen we can just allow
the magic behaviour from GLES 3 in GLES 1 and 2 as well. It shouldn't matter
what the internal value of the draw buffer is in GLES 1 and 2 because there
is no way to query it from the external API.

Reviewed-by: Kristian Høgsberg <[email protected]>

1 files changed, 6 insertions, 1 deletions

diff --git a/src/mesa/main/buffers.c b/src/mesa/main/buffers.c
index 6cbce9d5da2..b13a7af65a6 100644
--- a/src/mesa/main/buffers.c
+++ b/src/mesa/main/buffers.c
@@ -101,7 +101,7 @@ draw_buffer_enum_to_bitmask(const struct gl_context *ctx, GLenum buffer)
       case GL_FRONT:
          return BUFFER_BIT_FRONT_LEFT | BUFFER_BIT_FRONT_RIGHT;
       case GL_BACK:
-         if (_mesa_is_gles3(ctx)) {
+         if (_mesa_is_gles(ctx)) {
             /* Page 181 (page 192 of the PDF) in section 4.2.1 of the OpenGL
              * ES 3.0.1 specification says:
              *
@@ -111,6 +111,11 @@ draw_buffer_enum_to_bitmask(const struct gl_context *ctx, GLenum buffer)
              *
              * Since there is no stereo rendering in ES 3.0, only return the
              * LEFT bits.  This also satisfies the "n must be 1" requirement.
+             *
+             * We also do this for GLES 1 and 2 because those APIs have no
+             * concept of selecting the front and back buffer anyway and it's
+             * convenient to be able to maintain the magic behaviour of
+             * GL_BACK in that case.
              */
             if (ctx->DrawBuffer->Visual.doubleBufferMode)
                return BUFFER_BIT_BACK_LEFT;