mesa: fix error checking of DXT sRGB formats in _mesa_base_tex_format()

For formats such as GL_COMPRESSED_SRGB_S3TC_DXT1_EXT we need to
have both the GL_EXT_texture_sRGB and GL_EXT_texture_compression_s3tc
extensions.  This patch adds the missing check for the later.

Found when checking out https://bugs.freedesktop.org/show_bug.cgi?id=65173

NOTE: This is a candidate for the stable branches.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>

src/mesa/main/teximage.c

@@ -315,15 +315,17 @@ _mesa_base_tex_format( struct gl_context *ctx, GLint internalFormat )
       case GL_SRGB_EXT:
       case GL_SRGB8_EXT:
       case GL_COMPRESSED_SRGB_EXT:
-      case GL_COMPRESSED_SRGB_S3TC_DXT1_EXT:
          return GL_RGB;
+      case GL_COMPRESSED_SRGB_S3TC_DXT1_EXT:
+         return ctx->Extensions.EXT_texture_compression_s3tc ? GL_RGB : -1;
       case GL_SRGB_ALPHA_EXT:
       case GL_SRGB8_ALPHA8_EXT:
       case GL_COMPRESSED_SRGB_ALPHA_EXT:
+         return GL_RGBA;
       case GL_COMPRESSED_SRGB_ALPHA_S3TC_DXT1_EXT:
       case GL_COMPRESSED_SRGB_ALPHA_S3TC_DXT3_EXT:
       case GL_COMPRESSED_SRGB_ALPHA_S3TC_DXT5_EXT:
-         return GL_RGBA;
+         return ctx->Extensions.EXT_texture_compression_s3tc ? GL_RGBA : -1;
       case GL_SLUMINANCE_ALPHA_EXT:
       case GL_SLUMINANCE8_ALPHA8_EXT:
       case GL_COMPRESSED_SLUMINANCE_ALPHA_EXT: