Enable Buffer::invalidate*Data() on ES.

This smells fishy. The function _does have_ a no-op fallback
implementation to remove the need to wrap everything with an #ifdef
and/or extension check, so why I did exactly that everywhere?

src/Magnum/Buffer.cpp

@@ -195,7 +195,6 @@ Buffer& Buffer::setSubData(const GLintptr offset, const Containers::ArrayReferen
     return *this;
 }
 
-#ifndef MAGNUM_TARGET_GLES
 Buffer& Buffer::invalidateData() {
     (this->*Context::current()->state().buffer->invalidateImplementation)();
     return *this;
@@ -205,7 +204,6 @@ Buffer& Buffer::invalidateSubData(const GLintptr offset, const GLsizeiptr length
     (this->*Context::current()->state().buffer->invalidateSubImplementation)(offset, length);
     return *this;
 }
-#endif
 
 void* Buffer::map(const MapAccess access) {
     return (this->*Context::current()->state().buffer->mapImplementation)(access);

src/Magnum/Buffer.h

@@ -722,7 +722,6 @@ class MAGNUM_EXPORT Buffer: public AbstractObject {
             return *this;
         }
 
-        #ifndef MAGNUM_TARGET_GLES
         /**
          * @brief Invalidate buffer data
          * @return Reference to self (for method chaining)
@@ -744,7 +743,6 @@ class MAGNUM_EXPORT Buffer: public AbstractObject {
          * @see @ref MapFlag::InvalidateRange, @fn_gl{InvalidateBufferData}
          */
         Buffer& invalidateSubData(GLintptr offset, GLsizeiptr length);
-        #endif
 
         /**
          * @brief Map buffer to client memory

src/Magnum/Test/BufferGLTest.cpp

@@ -53,9 +53,7 @@ class BufferGLTest: public AbstractOpenGLTester {
         #ifndef MAGNUM_TARGET_GLES2
         void copy();
         #endif
-        #ifndef MAGNUM_TARGET_GLES2
         void invalidate();
-        #endif
 };
 
 BufferGLTest::BufferGLTest() {
@@ -71,10 +69,7 @@ BufferGLTest::BufferGLTest() {
               #ifndef MAGNUM_TARGET_GLES2
               &BufferGLTest::copy,
               #endif
-              #ifndef MAGNUM_TARGET_GLES
-              &BufferGLTest::invalidate
-              #endif
-              });
+              &BufferGLTest::invalidate});
 }
 
 void BufferGLTest::construct() {
@@ -348,11 +343,7 @@ void BufferGLTest::copy() {
 }
 #endif
 
-#ifndef MAGNUM_TARGET_GLES
 void BufferGLTest::invalidate() {
-    if(!Context::current()->isExtensionSupported<Extensions::GL::ARB::invalidate_subdata>())
-        CORRADE_SKIP(Extensions::GL::ARB::invalidate_subdata::string() + std::string(" is not supported"));
-
     Buffer buffer;
     constexpr char data[] = {2, 7, 5, 13, 25};
     buffer.setData(data, BufferUsage::StaticDraw);
@@ -365,7 +356,6 @@ void BufferGLTest::invalidate() {
     buffer.invalidateData();
     MAGNUM_VERIFY_NO_ERROR();
 }
-#endif
 
 }}