Fix readBuffer validation

The argument to glReadBuffer should be validated against
GL_MAX_COLOR_ATTACHMENTS, not GL_MAX_DRAW_BUFFERS.

Bug: angleproject:4766
Change-Id: Ic184934bd01e68b936266eae69543ae66fd68ea3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2252978
Reviewed-by: Kenneth Russell <[email protected]>
Reviewed-by: Jonah Ryan-Davis <[email protected]>
Commit-Queue: Kenneth Russell <[email protected]>

Author: jdarpinian

src/libANGLE/validationES3.cpp

@@ -1661,9 +1661,9 @@ bool ValidateReadBuffer(const Context *context, GLenum src)
     {
         GLuint drawBuffer = static_cast<GLuint>(src - GL_COLOR_ATTACHMENT0);
 
-        if (drawBuffer >= static_cast<GLuint>(context->getCaps().maxDrawBuffers))
+        if (drawBuffer >= static_cast<GLuint>(context->getCaps().maxColorAttachments))
         {
-            context->validationError(GL_INVALID_OPERATION, kExceedsMaxDrawBuffers);
+            context->validationError(GL_INVALID_OPERATION, kExceedsMaxColorAttachments);
             return false;
         }
     }